Why do you sometimes feel like you are falling when you are in bed trying to fall asleep or when you awake?

It is a hypnogoguic hallucination

Your brain is being awakened through the reticular activating system in the brainstem, however, you are still in a state of "sleep paralysis" so your muscle tone is basically zero. So, the mind awakes before the body awakes, you get this weird sensation. Totally normal, I might add.

I was told that this is the way your body goes into different levels of sleep, for if you notice usually after you jerk from the sensation of falling your body relaxes and indeed you fall into a deeper sleep

i really have no clue and this is just going to be a guess, but i think its because when you have a sortof dream and you fall and trip in it, i think your body feels like its falling because you're falling in your un/subconscious, but you're still halfway awake, so your body thinks you're falling for real when you're falling in your mind. i think it happens when you're close to exactly half sleep and half awake, it confuses your body a little bit so that you're body thinks you're awake if you're really asleep and vice versa.