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Why Did My Dog Suddenly Become Aggressive? Here’s 7 Reasons

Did your dog suddenly become aggressive to you, or another animal? Perhaps it became aggressive to someone else in your home? Veterinary Technician Amber LaRock looks at the most common reasons this happens, and when it's time to call a veterinarian.

Amber LaRock

Last Updated: April 27, 2022 | 8 min read

Aggressive Dog About to Bite Owner

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Did your dog suddenly turn to you aggressively with no warning at all? There are many different factors that can impact canine behavior. Some are more obvious than others, so it’s important to understand what may be causing the behavioral change.

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Aggression in dogs is not something to take lightly. Not only is this a concerning behavior for dogs in general, but it is especially worrisome in a pup that is usually friendly! So what could cause a dog to suddenly turn aggressive toward you or other dogs?

In order to address and change the behavior, you need to understand why it’s happening. In this article, we’ll discuss the details of canine aggression in our furry friends and help you understand why your pup is experiencing this sudden change in temperament!

Signs of Aggression

Small Dog Barking at Another Dog
There are many ways dogs show aggression toward other dogs and humans.

Before we discuss the details of this sudden behavior in dogs, it’s important to understand what aggression can look like in our furry friends. Aggressive behavior is very different from mouthing or biting behavior. Some dogs can experience mild grumpiness from time to time, which is not always a major cause for concern. So, what are the signs of actual aggression in dogs?

The most common signs of aggression in canines include:

  • Tense or rigid posture
  • Intense stare
  • Hackles raised
  • Growling
  • Showing their teeth
  • Muzzle punch, which involves the dog hitting you or others with their nose
  • Deep bark that sounds more threatening than usual
  • Snapping
  • Growling or guarding objects
  • Bites ranging in intensity
  • Biting and retreating
  • Biting and continuing to attack

While this is not always the case, most dogs show some warning signs before they bite. These warning signs are often one of the many different ways a dog may be trying to communicate. Some dogs may turn to destructive behavior by digging up the yard or stress-induced yawning to let you know they are unhappy, but aggressive warning signs look entirely different.

Many dog owners label their dog’s aggressive behavior as “sudden” or “unexpected” when really, they are missing many clues leading up to the action. The best way to stay informed on canine aggression is to educate yourself on the many possible signs. Staying informed can help keep those in your home safe, as well as prevent negative interactions with other animals.

Types Of Canine Aggression

German Shepherd Baring its Teeth
Growling, barking, and baring of teeth are the most common signs of canine aggression.

Before diving into the most common causes of sudden aggression, let’s first discuss the types of canine aggression you may see in your furry friend. Multiple examples of aggressive behavior can stem from these categories, making it important to understand the fundamentals.

Protective/Maternal Aggression: Aggressive behavior that stems from protecting their offspring or puppies, often coming from a female.

Territorial Aggression: Aggressive behavior directed toward an animal or person that is entering their territory. Their territory can include their home, their yard, or any other space your dog claims as to their own.

Predatory Aggression: Aggressive actions directed toward other animals that they are interested in hunting. This aggression can be tied to a dog’s prey drive and often results in a deadly strike.

Fear Aggression: Aggressive behavior that is fear or anxiety-based. This behavior can involve fear in new situations, environments, or meeting new people. This behavior is often due to an animal feeling cornered or trapped by its current surroundings.

Sexual Aggression: Aggression that is tied to mating behavior. This behavior is directed toward the animal they are mating with or other dogs that seem like a competition.

7 Reasons For Sudden Canine Aggression

Now that you understand what canine aggression can stem from, it is time to discuss the seven most common reasons behind sudden aggression in our beloved companions. Ranging from protecting their favorite toys to feeling unwell, your dog’s aggression could be pointing to something more!

Being Possessive

Small Dog Being Protective of Ball
Dogs can be very possessive over their space, toys, food, and even their owners.

Possessive aggression is one of the most common forms of canine aggression that pet owners experience. Possessiveness, or resource guarding, involves a dog guarding a specific object they claim as their own and becoming aggressive when anyone approaches it. This aggression can happen when an owner tries to take this object away or even when a person or animal approaches the area.

Not only can this occur when a dog is guarding their food or a favorite toy, but it can also happen when strangers enter their home. Dogs can become extremely possessive over their space, causing them to display aggressive behavior if they feel like their territory is being invaded. If aggressive behavior occurs when he is trying to guard a resource, it is likely due to possessive aggression.

Redirected Aggression

German Shepherd Growling at Another Dog
It’s common for dogs to be triggered by one thing, then direct the frustration toward another trigger that interferes.

Redirected aggression is another common form of canine aggression. Redirected aggression involves a dog becoming stimulated by a specific trigger, only to be interrupted by something or someone in the process. This trigger causes a dog to redirect its aggression away from the original trigger and turn it onto the person or animal that interrupted them. This reason is why it is so important to never interrupt a dog fight with your bare hands.

For example, many pet owners will receive severe wounds when de-escalating a dog fight. The owners often reach for their dogs to separate them, only to have one of the dogs turn around and suddenly bite them. This aggression can occur in otherwise friendly animals and dogs of any age or sex.

Pushing Their Limits

Child Playing With Beagle's Ears
You should always pay attention to warning signs your dog may show that his limits are being pushed.

Pushing a dog to the point of frustration can lead to aggression in some of our canine friends. Many dogs have limits to specific behavior they can tolerate and may become aggressive if those boundaries are pushed. Just like you and I may not tolerate a repeated action over time, our dogs are the same way.

For example, many dogs will offer a few warning signs to show that they are frustrated with a current scenario. A dog may snarl and growl at a child tugging on their ear, only to become overwhelmed if their warning signs are not understood. Pushing a dog’s limits may cause them to become aggressive, often leading to bites and injury. If your dog was not left alone as their frustration began to rise, his boundaries were possibly pushed.

Pain

Fluffy Dog Laying on Wooden Floor
Newly developed pain can cause dogs to show hostility toward their caretakers.

Pain can cause a dog to behave in many abnormal ways. Discomfort can decrease their tolerance in multiple situations, causing them to become aggressive in some cases. Not only does pain hurt, but it can be extremely stressful for a dog to handle. This is especially true if it’s due to an acute injury or illness. A dog may become aggressive when the painful area is touched or experience aggression due to a buildup of stress.

Identifying why a dog is in pain can be tricky. Pain in dogs can be due to joint conditions, lacerations, injured limbs, GI conditions, back injuries, and more. If you think your dog is experiencing a painful episode that triggered their aggression, it’s best to contact your veterinarian for further care.

New Medical Conditions

Chihuahua Growling at Human Hand
A medical condition that presents itself can generate a mood shift in your dog.

A new medical condition can be extremely stressful for a dog to handle. Medical conditions can trigger sudden canine aggression, especially if it leads to changes in daily routine. Not only can the onset of new symptoms lead to heightened anxiety, but a change in their structure can further aggravate a stressed-out pup. A change in routine can involve new daily medications, more trips to the vet, and any decline in their overall health.

The onset of a new medical condition can not only lead to new canine aggression but may even provoke aggression in an already grumpy dog. If you think your dog’s sudden aggression is tied to a new diagnosis, we suggest speaking with your vet about ways to offer comfort.

Changes in Their Environment

Small Black Dog in New Home
Moving to a new home, having visitors, or staying at another home bring about stress and aggressive behavior.

Many dogs thrive in a structured environment. Our canine companions find comfort in a general routine, leading to stress if there are any sudden changes in their life. A stressed-out dog is more likely to snap in certain situations, opening up the door for behavioral changes. Major shifts at home can cause a dog to react in ways it wouldn’t have before and may even display aggressive behavior as a result.

Dogs can experience stress due to switching homes, the sudden absence of an owner, new people in the home, new pets in the home, and more. If your dog’s aggressive behavior began after a change in their routine, this could be the cause of their aggression.

Aging

Senior Dog in a Field
An older dog is prone to more complications that arise with age. Be patient and gentle when they begin this behavior.

While age is not a disease in itself, it can bring many life changes. A senior dog is more likely to experience chronic pain, new medical conditions, and even heightened stress due to environmental changes. Each of these complications can lead to sudden aggression on its own. Any one of these factors can cause noticeable behavioral shifts.

If you notice heightened aggression in your dog once they entered their senior years, it’s best to discuss this occurrence with your veterinarian. Your vet can potentially diagnose any condition that is causing your pet distress, as well as offer ways to bring them comfort in their senior years.

Addressing Your Dog’s Aggression

Pomeranian Baring Teeth at Camera
Though some aggression can seem cute, it is substantial to promptly and correctly address the behavior to avoid injury.

The sudden onset of aggressive behavior in your dog should always be taken seriously. Even more important than acknowledging sudden behavioral change is approaching it in the right way, as a negative reaction can worsen the situation. Let’s discuss the recommended steps you should take in addressing your dog’s aggression.

Find The Root of the Problem

The first step you should take in addressing your dog’s aggression is trying to get to the root of the problem. Examining your dog’s life for any potential triggers can help you solve the problem and even prevent future aggression. It is rare for a dog to experience a severe behavioral change without a trigger.

Speak With Your Veterinarian

If your dog is ever experiencing sudden aggression, we always suggest speaking with your veterinarian. The only way to rule out an underlying cause is by seeing a professional and discussing your dog’s history with a vet that knows them. If your dog’s behavioral change is due to an undiagnosed condition, you may never resolve the issue on your own.

Seek Professional Training

Canine aggression should always be taken seriously. Even the sweetest of dogs can accidentally harm us. You’ll need to seek professional training in situations involving aggressive behavior. Professional dog trainers are also skilled in identifying potential triggers in your home. This will help make it easier to get to the root of the behavior.

Avoid Potential Triggers

If you can identify a specific trigger for your furry friend, it’s critical to make it’s life easier by avoiding the situation when possible. While not all causes are easily avoidable, some canine triggers are. For example, if it seems like your dog is triggered by new animals entering your home, it may be best to avoid welcoming in any new pets.

Final Thoughts

Sudden aggression in dogs has been linked to many potential factors. After reading this article, you’ve likely been able to identify at least one reason why aggression may have become a newly developed problem.

If your pup suddenly became aggressive, the first thing to do is to consult your veterinarian. You’ll want to make sure it’s not health-related. From there, you’ll want to work through the steps above with a canine behavioral specialist. It’s critical to address the behavior and work with your dog to correct it do it doesn’t continue.

The information provided through this website should not be used to diagnose or treat a health problem or disease; it is not intended to offer any legal opinion or advice or a substitute for professional safety or care advice. Please consult your health care provider, attorney, insurance expert, or product manual for professional advice. Products and services reviewed are provided by third parties; we are not responsible in any way for them, nor do we guarantee their functionality, utility, safety, or reliability. Our content is for educational purposes only.

Leave a Comment

10 Comments

ANA Barreda

May 10, 2022 at 12:19 am

Hi,I own a border Coli and he is 1 year old. He has became agresive with people and been biting people three times. I am very sad about it and very worried.
He has never been like that until the last two months. We think is that we took another dog we have out of the house and now he is on his own. Could that be a possible reason to him become agresive?

Reply

Michelle Schenker

May 10, 2022 at 1:29 pm

It is possible - any change in routine or environment can trigger changes. You should consider talking with a local behavioral expert or trainer to see if you can resolve this concern.

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Lorraine Sacks

May 9, 2022 at 10:59 pm

I have a 6 yr. old Basett Hound. He has been a very sweet, playful friend. Then a few months ago, he stopped playing and became very aggrsive to me. But mostly at night. I have taken him to his Vet that has been his Dr. since he was 7 weeks old. Because this happens at night she has not seen this happen. I took him for x-rays, nothing. I took him for a CT scan. I do not know what if anything that showed yet. Just did that yesterday. I have been bitten several times. He also went after my Cat for no reason. I am a affraid of my Dog. And I do not know what happened. I am so affraid that I will have to have him put down. I hope you read this, and with God's help someone has an answer. Thank you, Lorraine

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Michelle Schenker

May 10, 2022 at 1:27 pm

Hi Lorraine,
Perhaps consider talking with a local behaviorist who can come at night or review videos that you make of this behavior, so they can help you out. Wishing you and your dog all the best.

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Zola

April 6, 2022 at 10:29 am

Hi there, I own a rottweiler male at 1.5 years of age. My dad alongside the other males barely spends time with him due to work etc. My mother and I recently started walking our shepherd and our rottweiler and we moved about 2 months ago to a new bigger place. My rottweiler pup has been acting really strange these few weeks. He snapped at my father when my father tried to put his collar on but I'm not sure if it triggered an old memory at the vet (the vet was a male) but even after that he continues to be tense around my father sticking to me only and does not listen to commands my father gives him. He also snapped at my dad when my dad tried petting him on the head. We consulted a friend who is also a dog trainer and she said there is a possible change in the hierarchy caused by my mum and i walking him(the dog). Continuing to seek help my dad talked to one of his friends who was a soldier in the army and his solution was putting a muzzle on the dog and hitting him to show him his "place". We as a family however do not believe in any sort of physical violence when it comes to animals. My rottweiler then attacked my grandfather who pet him on his head and now refuses to listen to him. Rottweiler pup stares my dad and grandpaps down and slowly circles them he also tracks their movement from the corner of his eye. I dont know if the problem is male related, my neighbour (male) doesnt really like our dogs i dont know if he perhaps done something to our pup.t The most recent incident was when he snapped at my mum . We've spoken to the breeder where we got our puppy from and he says the dogs mother exhibits the same behaviour. My rottweiler hasnt attack me or growled at me before he did snap once when i was petting his neck but immediately apologized. I have no idea what to do next we are looking for a new home for him and our last option is to put him down. Please please offer ur opinion because my rottweiler is truly caring when it comes to me but not everyone else.
Please reply

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Michelle Schenker

April 8, 2022 at 4:16 pm

This is an extremely personal decision, but you might consider working with a local behavior therapist or trainer to address your concerns. This is not something that can be resolved via the internet in our opinion - it requires personal attention and customized training for your unique pet and situation. We are so sorry to hear of your troubles and wish you and your dog the best.

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Sherry

April 4, 2022 at 8:18 am

Hi, I have a 15 month old Un-neutered chocolate Labrador. He has been trained since day one at 8 weeks to crate with a stuffed animal . I would put my hand in his food bowl to avoid food aggression. His food bowl was in his cage since he came home . he never showed any aggression till he was about a year old . I had given him his food closed the cage as I always did but I needed to get his water bowl and when I opened the cage he gave me a lowered head with a dead on stare followed by a growl. Which caught me off guard and I said no ! And closed the cage because I was unsure how to handle the situation. I am presently taking on line classes for dog training . However I have done so far what I have learned . I removed his food bowl from his cage because I thought maybe it was his space he was guarding . So I placed his bowls in our kitchen where I walk and move around him while he eats to show him I don't want his food . I have even called him away from his meal time to give him a tasty treat to show him better things come when he's not guarding his food . I thought we were making progress (but a side note when we are eating he wants to sit beside us and that's fine but he starts to drool so we ask him to go lay down. ). He is a very loving smart dog that I have trained to do many tricks I play hide and seek with him. Of course I'm the one hiding and he finds me . We take long walks on the state game lands next to us . So he is a very active well balanced dog . He goes most places with me in the car and sits quietly in the car . So I'm confused on his actions . Yesterday we were eating he already ate his just faster then we did . He came into the living room as we were eating and sat next to his dad(husband) and dad made a comment saying look he has not growled today and on the spot he started to growl and took a stance with hackles up( hairs on his back) ears pinned back and dad said no that's bad . And before I knew it teeth were showing and dad had grabbed him by his color and throat to keep him from biting him all this time it sounded like a fight going on . Husband manage to place him in his crate for our safety and time for him to calm down from whatever reason he has become aggressive. It breaks my heart because I know how living he is . He has never been with anyone but us after we brought him home. He does play with my daughters dog a 11 months old female German Shepard . Now she gets pissy at time and try's show her dominance over him but he correct her . I was just wondering if you have any ideas as to why he might be acting this way towards us in our home no one else around . . I have done everything so far that I know . Thanks for your time .

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Stacie

March 31, 2022 at 1:09 pm

Hi, my Nearly 2 year old neutered male has gone into a few fights and it’s really stressing me out so I wound weed if you had any advise.
1st time he got into a fight with a dog he plays with most morning (wasn’t sure how it started)
2nd time he got into a fight with a puppy much younger than him he plays with every afternoon. Not sure how it started. He has been fine since seeing the dog but I’m scared it could happen again.
3rd time he got into a fight with a dog he plays with over a ball. This has never happened before but instead of a growl he went straight into attack.
4th time he was stood on lead and a dog passed and sniffed him and he snapped, luckily I pulled him back before he could make contact.
5th he ran over to a dog near our home aggressively barking but didn’t make contact.
He is usually the friendliest dog and really submissive. There doesn’t seem to be any triggers or pattern. There seems to be no warning just aggressive fight where they need separating.
What used to be my stress relief walking the dog is now the most stressful, worrying if he will go for a dog, do I keep him away or does that create a problem.

Thank you for your time and help.

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Eric

February 5, 2022 at 12:48 am

Thank you very much over the explanation.the reason been that training a Dog is a patient method.but Given you does sign off aggressiveness is a credit to the dogs owner.not being friendly to people.here I rest my case

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Deborah Morse

January 21, 2022 at 9:57 pm

My chihuahua is 6 years old. Unspayed female. She has toy(sock monkey) that she has had since she was a baby.
I have not ever noticed her going into heat. Except for sniffing the grass and “marking “ the spot. She will also carry the monkey around everywhere she goes.. She has never been aggressive. That behavior ends. Then over some time, it happens again.
My boyfriend and I have raised her together. Now in separate homes.
We have our individual ways, and she is used to this.
He was in the hospital for the last 3 months and is staying with me right now.
She has always laid under the bed covers with him.. Now she has the monkey with her under the covers next to him. But is agitated now towards me.
The one thing that happened in the last week or so is. I ran out of her dog food. I got some from my neighbor. She wouldn’t eat much of it. It was a few days before I could get to the store, because I was sick and he can’t hardly get around.
So once I got to the store, they didn’t have the food she likes.(very picky) but I bought something else.
After that I noticed her looking at me with nasty eyes. It was only another day or so, I got her the right food.
She has turned into Cujo. I don’t know what to do

Reply