Swimming in air

What I expected to happen was...:
So I was closing the trap door on my house expecting it to just close on top of me

What actually happened was...:
But instead I was pushed into the swimming animation when there was no water

Steps to Reproduce:
Take a 2*1 doorway. Then make a 1 tall tunnel on the other side. Then place two trap doors on the wall. one going up, the other down. Walk into the doorway, close the top trap door, And you should be pushed down and able to walk trough the 1 wide gap.