I don’t think dogs go off for no reason.
It may not be readily apparent but there is always a reason.
It would only be conjecture based on the info given.
Your dog was sensing something you weren’t...
Was there alcohol involved?
Is your neighbor a jerk, overbearing, loud, aggressive?
Could DOG have thought you were being threatened?
What was the neighbor doing at the time your dog became aggressive?
What did DOG do, start barking? hackles up?
You say you grabbed his collar, it appears that he hadn’t at that point made a charge at the neighbor?
Was DOG trying to get at the neighbor once you had hold of his collar?
Did DOG have contact with the neighbor at a later time?
If so, what was his reaction?
The neighbor simply turned his back, before that it was all petting and tail wagging, licking the neighbor just normal puppy until the neighbor turned to walk away.
Neighbor hadn’t even realized anything was amiss, I was just lucky that my reflexes were fast that day.
No Alcohol, neighbor is “Dog Poor” (he loves his dogs), the dog did not bark, just short growl his hackles raised (jekyl / hyde).. immediately before he went for the back side of my neighbor (when he turned his back), after I grabbed his collar he didn’t have any other choices (he looked like he was sorry).. dog was gone that afternoon so no follow up is available.
TT