I did this testing in the Nether (also happens in the overworld and The End).
When going through a tunnel that are 2 blocks high, the player will likely fall through the blocks or a chance to go in crawling/swimming animation.
This happens because when the player dismounts, it will put the player on top of the boat, rather then on a block next to it.

To reproduce:
1. Make a tunnel that is 2 blocks high and wide on the ground.
2. Get a Minecart or a boat
3. Now crouch to dismount once your inside the tunnel.
4. You will then fall through the blocks! (Or getting into crawling/swimming animation)

Video evidence is shown here:
(It might be a duplicate, but haven't seen any reports about this.