One time on Burning Paris inside the hotel bridgeway that goes over the fountain area, a crawler came from a vent and landed on an ally's head, but because it had nowhere else to land besides the player's head it instead dealt tons of small ticks of damage in a really fast period, killing him in about 3 seconds.
The damage seems to scale depending on how much difficulty the zed has getting down. I've also seen the 'goomba stomp' happen sometimes, where if a zed falls from the ceiling onto your head it'll deal some damage for every time it collides with your hitbox.
I see gorefasts and gorefiends do it a lot, but sometimes Scrakes/FPs do it as well. It usually happens when a zed is trying to get around a large object like a Bloat or a crowded doorway, causing the zed to bounce onto a Crawler's back and ride it like a surfboard.
