Against who? Whose fault is it?
It would be the fault of the owner of the horse for continuing to let people ride it, assuming it happened again and the rider was not informed of the horse's history.
Again, IF it wasn't a typical "horse shut off" case and there was some evidence that the horse deliberately jumped onto the guy. (Doesn't take much.)
I don't mind horses that shut off as much as I mind horses that rear, though. If one makes a habit of that I strike him off my list. Too dangerous.