Every item in the game has a chance of being destroyed each time it’s used. For items like potions and scrolls, that chance is 100%, while for items like weapons, armor and shields, it’s currently 0% (which may change in the future, as at least non-magical weapons, armor and shields do tend to break). For Boots of Speed, it’s currently set to 2%, meaning that, on average, they will break and be destroyed every 50 times they’re used. Hence, try not to overuse items in your inventory, as they may break when used and be lost permanently.