Without seeing the entire situation unfold it's tough to say, but here's a guess. You say he has issues with horses already. Perhaps his issues are fear-based. Fear-based could show up as: "the best defense is a good offense" (he attacks and drives them off first). If fear-based, the ability to flee would also be important to him. He can hardly walk right now, much less flee, even within his stall and certainly when tied. Perhaps he is just trying to put up a good offensive while he is incapacitated. Keep him as isolated from horses as possible until he gets better so it doesn't become a stronger habit. Then, after he's completely healed, see if this new behavior subsides.