Why the heck does my body feel like it's tripping sometimes when I sleep?

So I'm about to snooze when all of a sudden before I drift off it feels like I missed a stair in my bed and I start to fall until I'm forcefully woken up by the shock. Like, WTH?! It's not like a dream where you fall off a skyscraper, it's more like before you start to dream when it happens.