Bud has barreled past me when I was leading him through a gate. He smashed my elbow on the gate but I didn't break anything.
It's easy to see how an accident like this could happen. If I leave a stall door open and the horse starts to walk out, I'll block them with my body. At an unfamiliar place, like the competition that this vet was at, a spook is a likely occurance and probably what happened.
Any horse that's already decided to bolt ("I'm gonna go for it!") is dangerous to walk in front of, you're right. They're too big to stop if they don't want to!