Clients say this so often I almost expect it. Dogs do all sorts of things that seem to come from nowhere, but with analysis they are actually behaviours with obvious causes. Here are some examples:
1. Baby Puppy bites Vet when having his mouth examined: Not many of us relish the idea of a dental checkup because it is very invasive and potentially painful. Along with this we can add in for the puppy the fact that this is a stranger handling him, in a strange environment, without his familiar humans (because of social distancing at present), plus any previous examinations of eyes, ears, temperature taking (eek!) and vaccination. If we look at all the stressors leading up to the mouth examination, the poor baby just couldn't deal with any more and lashed out. He's not a bad or aggressive puppy. He was frightened.
2. Older Puppy bites trainer (!) when having halter fitted. I am expecting to see a lot more of these COVID-puppies. There are so many who came into their new homes just before lockdown, or were adopted/purchased just after lockdown. These youngsters then missed out on Puppy School, playtime in parks, social interaction with human friends and family, and doggy friends and family. Through their critical learning period they had no choice but to miss out on these lessons. Some dogs cope very well, and some do not. Some young dogs have decided they are in charge of their home and their humans. They don't understand other dogs and are wary about humans they don't know. Add in that their humans often don't have the skills to handle the situation (something we all have to learn, we aren't born with it) then things just get worse. If a dog never learns to deal with "No", then when finally they are thwarted they will often lash out. Again, this is not a bad dog. This is a dog that needs to understand that she isn't the one in charge.
3. Adult dog lashes out at another dog. Some dog owners have seen this for themselves. As trainers we see this over and over again. To begin with, dogs have excellent long term memory. If they have been attacked and/or bullied by another dog in the past, they learn to be wary of dogs they don't know. It takes just one experience to colour their view of the world. Even a scary moment in puppy pre-school can do this. Then when we add in more stressors such as the dog being on leash when the strange dog (or human) runs up to them, or their humans are not at home when the stranger comes onto their property, we have a potential for dramatic responses. Again he's not a bad dog. He made a mistake based on past experiences.
A dog on leash has lost their "flight" option so they can't run away if they want to, so they defend themselves. A dog dealing alone with an intruder can't check with their human as to whether they allow the incursion or not, so they act like a dog and defend their property.
All of these examples of dogs making mistakes can be helped through training. The baby puppy can learn that handling of their faces and feet is not a bad thing. They can learn to trust that their humans will keep them safe. The older puppy can learn that "No" is OK and to look to their human for leadership and trust the human's judgement. The adult dog can learn coping skills and obedience exercises that will assist them and their humans to manage difficult situations.
Training is essential for our dogs of all ages to help them live comfortably in Human World. If we can teach them to listen to us, to trust us, and we learn to make safe and sensible choices for our dog's safety, then many of these "I didn't expect this" moments can be avoided.