Maybe luck or low speed falls. Using your hands may be instinctive, but it is a good way to break your wrist or arm. If you put your arm out to stop your fall, your hand will stop but your body may keep moving due to inertia, and it may be a direction that you can't stop and load your joints in directions they were not meant to move. If you are able to keep your hands and arms in, you protect them, and hopefully are able to roll or absorb the blow over a greater area.