Sasuke had watched Shiro resting on the broom for a solid few seconds, wondering whether or not this would end with him stumbling forward or falling flat on his face naturally. He goes to walk away and let him rest in such a ridiculous position, but stops after a few steps, because ultimately, he feels like maybe he should... help him get back to whatever work he was trying to do.

So he does what one would expect.

He goes to knock the broom out from under him.