When you enter a minecart, the camera gets stuck just below the body of the cart, and puts your model in the swimming position.
It is really hard to see the actual model of your body, even when using F5 camera modes, as the camera is stuck and does not shift backwards in third person. However you can observe the swimming position on your model from the inventory .
The same thing with the player model happens when you step into the same block as a trapdoor and then open it above you, as in MC-141824.
The same thing also happens when standing in 1-block tall water and placing a lily pad in the water source you're standing on, as in MC-667