Oh, it wasn't the harness that triggered the dog.
The dog would simply go from good to bad for no reason. Even if he was just standing there happy as could be, he'd go stock-still and lash out unprovoked.
He had serious problems that were not related to any abuse, so far as I could tell. He was very young and the owners had another dog who was perfectly well-adjusted.
They just had the misfortune to get a brain damaged animal. :-(
Yeah, I happens. Scary. Sorry about your arm but I'm glad you were able to get away.