So when you use a riptide trident in a minecart, it moves the minecart, and you go under the minecart kinda, but it looks like you are still in the pose when you use the riptide trident. I think people would exploit it at some point when the bug is leaked (idk if that will happen though, just guessing). Here is an image of the bug in action:

I still don't know why this happens, but maybe you and the mojang team can fix this bug. BUT if the bug was already reported, I am ok with that. I guess I found my first bug.