Minecraft wiki entry for Crawling, associated text: Closing a trapdoor on the player
Closing a fence gate when it is at the same altitude as the player's head
Using a piston to push a block into the player
Exiting swimming mode in a one-block-high space
Exiting elytra mode in a one-block-high space
Having a tree grow over a player
Throwing an Ender Pearl into a one block space
Having a shulker box push the player down
This normally makes the player sneak instead of crawl, but when it is above a slab or a not-full block, then the player crawls.
Getting off a pig with a saddle if it's under a one-block-high space
Having a boat land on the player's head