Oh, that's happened to me before too, it's not all that uncommon. One explanation is while drifting off to sleep you suddenly catch yourself and become aware of your body again, and in the moment of disorientation feel like you're falling.
I've been sleeping more deeply lately, the last few dreams have been action dreams. Well, one or two were simply exploration. A bunch of cityscapes, and one that went from an artic ice cave to a more temperate cave to a nice park in some woods by the ocean. I love exploration dreams, the locations are always so atmospheric!