Don't watch the video if you're squeamish. The horse in stall 10 apparently panics in the gate, rears up and goes down.
The way a horse's poll is structured and the way a bone in the top of the head is a little "free" to move rather than being fixed means that if a horse hits the top of their head at a certain angle they can die - even from a mild injury. (Horses have died from being smacked on the head). It's just a weak point in what's normally a very, very solid protective structure.