I would like to point out though that well trained, well socialised, confident, happy, chilled out dogs do not EVER 'just turn'..
Not a thing.
Dogs are living beings though, they can be scared, they can be in physical pain and most commonly.. they can be mishandled and mistreated to the point where they no longer behave predictably.
One of the classic and most dangerous things many people do is punish their dogs for growling - ooh BAD DOG - and typically then carry on doing the thing that triggered the dogs discomfort (grooming, taking away food, grabbing collar to physically move the dog).
This teaches the dog that 'saying stop does not work' and so the dog stops warning people about how he feels... but still FEELS uncomfortable.
In every case I have ever come across of a dog 'just turning'... I have found a dog who was in pain, a dog who had been punished into not communicating, a dog who has been tolerating unpleasant treatment for years and has had enough, giving warning signs that were never heeded... I have never ever come across a healthy, happy, well trained dog who has 'just turned' and nor have any of my colleagues (of which I have hundreds thanks to social media and doggy behaviour conferences!).