The player will start crawling if its head is in a trapdoor, fence gate, or even a block pushed by a piston.
Linked issues
is duplicated by 78
MC-147757
Pistons pushing blocks ontop of the players head puts it into swim animation
Resolved
MC-147794
Eye height collisions can push into solid blocks
Resolved
MC-147826
Trapdoors force users into a swimming position
Resolved
MC-147828
Swim Animation Starts when Under a Closed Trapdoor
Resolved
MC-147864
Trapdoor forces player under it when closed
Resolved
73 more links
This is almost certainly intentional - there's otherwise no real easy way to enter crawling mode.