The player automatically jumps when pressing against a block while in water

I was playing around with my water bucket then my teacher called me away and when I came back I was jumping on the stair next to water without pressing space (jump).

To reproduce make water flow next to the stairs and stand halfway in between the FLOWING water and bottom step of stairs.

It can also happen with still water if a move button is held. It also affects non-player entities that extend EntityLivingBase, most notably armor stands.

It also happens when walking into flowing water against a solid block (you'll end up on top of the stone bricks):