Push box is a thing which prevents the characters from occupying the same space; it pushes the opponent if you get too close.
Think of it as the space your character takes up.
There's a bug with all push boxes, but it's most noticeable with whirlwind, where they're not working quite right. So often, when whirlwind should push the opponent, then land infront, it instead doesn't push them and crosses up.
So basically it crosses up waaaay more than it's supposed to right now.