If he was just a stall biter, that's one thing. But if only 2 people could ride him, that points to a more serious temperamental problem. It doesn't sound like a normal "fall over the fence" accident, but without knowing more facts we can't say for sure.
But if, after that accident, he had dumped another rider and then jumped on top of him, there would be a liability problem. Unless the rider knew of his temperamental problems and his tendency to jump on people . . . then there might be an assumption of the risk. But it would be a jury issue for sure, and trials are expensive . . .
OK - Two people at that barn could ride him. That doesn't mean they were the only two people in the world capable of it, it's a rule that they made. So he was not a lesson horse. There's lots of horses boarded at stables that are not to be ridden by anyone other than the owner or a particular trainer, most horses are not just 'available for the taking' by any of the lesson kids.
He was apparently a biter, but that isn't what got him in trouble.
I don't know what about the accident doesn't make it sound like a typical refusal accident where a horse skids or jumps through a fence.
Quote: "One day, not long before I got there, Patrick was taking Junior around the ring, over some basic jumps. They approached the Wall, which was a bricky-looking jump about three feet high.
Junior decided at the last minute that he didn't want to clear the Wall, so he stopped short and let Patrick go over without him. After Patrick landed, then Junior decided to jump after all.
He landed on Patrick, and after that there was only one man who could ride him. Ugh."
I know you're looking from a legal point of view... and as someone who jumps horses yourself, I'm particularly surprised at your take on this.
Xena... you've given us quite the side issue to discuss here :~)