You didn’t. Even the most gregarious and always-loved dogs have bad days. When I was a kid, we had a dog who was so friendly to everyone- except men wearing baseball caps. He was abused too. His reaction was to cower, pee, and then run instead of aggression. But it could have been and wouldn’t really be his fault.
Sounds like you were a good dog owner and good to Danny- you knew your dog, you knew his particular issues and mitigated them as best as possible.
We’re teaching our kid both for her safety and for the dogs who have issues through no fault of their own. I would hate for carelessness on our part to result in someone’s pet being killed because it felt there was no other choice but to bite and freaked out.
I’m not entirety sure how heavy the dog was- that is an awful lot of floof!