Aggression can and does happen as a response to illness, injury, or chronic pain. I'd say it's a common trigger.

But more often than not, aggression "out of nowhere", is in reality quite obvious and predictable if you know what you're looking for.

There are all kinds of signs of aggression and discomfort that dog trainers see that escape the notice of pet owners. And when they are discussed and attention is drawn to those issues, the pet owner often gets extremely defensive. It's a tough thing to deal with as a dog training professional.

In addition to the signs being there but unnoticed, the act of punishing the precursors to aggression like growling makes those precursors go away.

Without the precursor to aggression (growling) other critters (like humans and human children) don't know to stay away. The dog freezes, noticeably to a dog trainer, but this alternate behavior goes unnoticed by the pet owner, and the dog aggresses "out of nowhere".

I'd say between the 3 of these situations, you have pretty much 100% of the dog bites covered.

The number of crosswired, bad seed dogs is small, and these kind of bad dogs rarely "turn"; they're just kind of scary and bad. If more people knew more about reading canine body language and handling dogs most all of these issues would be nipped in the bud rather early.

Thanks for drawing attention to physical malady as trigger for canine aggression. It's a truth; important to know for the pet dog handler.

Peace~