Why does it feel like we fall sometimes while sleeping?

Sometimes, although rare, I feel like im falling right before I fall asleep and jolt awake. It starts off at when I go into a deep thought and dont realize that im thinking weird things, although i am still awake but really tired. so I would think of something like walking with my friend talking, and then I would trip over a rock in the thought and break through the floor, falling helplessly and it seems really real (I call this "falling" asleep). Does anyone know the cause of this and why it happens?