Some good points about helmet fighting being made here, for sure.
The first guy goes down with one punch, so motorcycle guy probably figured it was going to be an easy day. But then his buddy appears, so it's probably worth taking the helmet off so that you can breathe and move a bit easier. Since the visor is up, it's going to be that much easier to grab and yank around... which it looks like the first guy attempts to do. He does gently put his helmet down, but it could have been a pretty good weapon to stop swinging around.
My favorite part is the girly little slap he gives the first guy after cleaning his clock.