Apparently it's because of a discrepancy of brain and body energy/activity. Like, you just wake up and to do something that costs you energy, like a exercise, but you're tired because you barely slept the other day and when you come home you go to take a nap as fast as you can. So your mind or body is tired but the other thinks It isn't (generally your body is tired and your mind isn't, mine was but It thought It wasn't I guess because I drink too much coffee lol), and It makes for a discrepancy in equilibrium, and the closest thing your brain can correlate lack of equilibrium with is tripping/falling. This was happening to me once almost everyday because my sleep is shit lol, I would lay on the bed and in five minutes just by relaxing and not even fully falling asleep this shitty "dream" or flashing visualization/sensation of me falling would happen. I am not a expert at ALL lol, but I searched about It at the time and this information was the closest to what I was feeling and what seemed to be happening.