I would say the only thing possible that is not listed is somebody messing with him when you can't see. I'm not sure of your situation but people can mess up dogs by hitting kicking bashing the fence etc.
Had a similiar situation to this with a blue heeler I had. Had never been aggressive then out of nowhere he tried to bite my FIL. Sent him to a new home, behavior problems dissapear. We suspect the neighbor, who had a history of teasing and doing mean things to dogs, was screwing with him.