My 4 year old GS has bit 3 people. Each situation has been the same thing, he was in the fenced yard, and one 12 year old boy reached down over the fence to pet him, and he jumped up and left teeth scrape on his chest. Another time a boy hid by our fence and he nipped him in the back leaving no mark. Today he bit a girl he knows as she reached down into his fence to pet him. He has fence line aggression if you can call it that, but when people are in the yard or the house, he has never shown any type of aggression to anyone. He barks at the door, and barks at anyone passing the fence line, even people he knows, but once inside the fence he is totally fine. I am really nervous about todays event. I am taking him back to his original trainer, and meeting with his vet tomorrow, any suggestions would be appreciated.