After being hit with the knockback enchantment anti-hack (I'm guessing) puts you back to where you were originally. It gave me a head ache D: I am hoping this can be fixed.
The reason it does that is because you're getting knocked back far and quick enough to where it thinks you're flying so it pulls you back to where you were. It's really annoying when you're sprint hitting someone and you have a KB sword as well, that's when it's the worst. I think it's NoCheat but I'm not sure. I'm guessing the developers will have to fix it because it happens on all servers.