We had a black lab. He was well socialized with both people and other animals. Other that one time when a homeless man threw a punch at my brother, I never saw him display any signs of aggression towards a human and then, he only growled and got between the man and my brother. He was especially submissive and careful with kids.
However, after about the age of 5, he suddenly became intollerant to any other dog. Literally, he used to be the center of the pack playing on the beach to trying to take on any and all comers. We had to seclude him from any dog contact.
I never could figure out what happened, literally overnight he went from loving dog company to hating every dog in the world. We considered putting him down but he was fine around people and lived out the rest of his life without incident.
He even loved the mailman!
The “getting between” maneuver is very common dog behavior, especially in a pack situation.
It serves to “defuse” an oncoming battle between two of its pack mates.
That’s why I stepped in as a “surrogate dog” in PetSmart.
The Husky recognized it for what is was and laid off his attack.
Any number of factors could explain the “sudden” aggression.
Some kind of subtle pain, shifting hormones [even in neutered dogs] and it could simply be put down to becoming an “old crotchety coot”...:)
I adivse *everyone* who has a dog who “suddenly” displays abnormal behavior to have FULL tick panel run.
Tick borne diseases account for a very high percentage of “sudden personality changes”, as can elevated liver enzymes and thyroid issues, oddly enough.
Bless you for keeping the old guy around, anyway...:)