Likely resulted from the fix to MC-129863/MC-131116/MC-130572/MC-132482.
This can happen in many different cases:
Placing a lily pad within yourself (MC-667)
Closing a door on yourself
Closing a fence gate on yourself
Having a piston push a block into your feet
Solid plants growing within a player (e.g. bamboo, trees)
Intersecting a falling block after it lands
Having a piston push a block into your head (likely intended - especially if the pushed block comes from above, and makes for easy entrances to one-block-high passageways)
Using an ender pearl to enter a one block high area (likely intended - stops you from suffocating, and sounds very useful)
Closing a trapdoor on yourself (likely intended - makes for easy entrances to one-block-high passageways, assuming the trapdoor is waist height)
It would be best if all intersections with the top half of the player still caused them to start crawling, but all intersections with the bottom half would not.
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Relates to MC-147264

Caused by the exact same bug as MC-147264, this is technically a duplicate just under different circumstances

Trapdoors still cause the behavior, not to mention it isn't crawling, just "swimming" in air, it's way too bizarre to be normal behavior.

It's still intentional behaviour.

The only intentional behavior is when it is to prevent unintended suffocation. That is not an issue with trapdoors at all, so it doesn't fit the intended criteria for "crawling" to occur.

Has Mojang officially stated that crawling is intended for non-suffocation-prevention cases, such as trapdoors (most obviously used for End Gateways)?
Well, I just wanted to agree with the oddness and awkwardness of the trapdoor case.