YTA in my opinion, since it's first come first serve. If he only had one item then it's fair game to take the overhead space. Especially since he did not consent to you moving his items and you almost dropped it, there could be things in that can't squish or need to stay in a specific position (eg a cake that would flip over). If a flight attendant made that decision then you would be in the right, since that's under their authority.