If turned out with others, that would be my thought too, that someone else is stepping on it. Or kicking and catching it somehow. In fence or on something in the stall. Or if the mats in the stall are funky, I've had horses get a shoe caught and pull it, but only once, and we figured it out pretty easily (shoe still caught on the mat!). Or perhaps he is just "special"? I've had one of those. Broke a shoe in half, still nailed on, but shoe split at the toe. He also rolled up hill.
Most of the time another horse is doing that. However, I had a mare who was springing the lateral branch of her hind shoes in her stall at night. Turns out she was rubbing her butt on her door and pushing the door away from the wall (the door was secured to the wall on a track with a roller ball and she pushed hard enough to bend the roller ball and allow the door to be pushed away from the wall). She was then putting her hind feet out into the aisle and getting the ends of her hind shoes caught on the metal track that ran along the bottom of the door and either springing her shoes or pulling them off. She also bent the metal corner plate that holds the door to the wall on the latch side of the door. We ended up having to reinforce the roller ball and corner plate with thick steel and add another latch to her door on the rollerball side (not the latch side).