Do you think that such behaviour from a dog is an innate tendency which careful and appropriate treatment and upbringing from the "owner" can negate almost fully?
Take cats for example - they can't be trained obviously and that's not the point I'm making. A cat can quite happily be stroked and tickled for ages but then go into a natural grab/scratch/bite mode without any malice, since this behaviour is pre-programmed from birth.
When a dog "loses it", do you think that it too is just returning to some natural instinct?