Remember, Carmello was still running with his thug posse as recently as a year ago (haven't heard anything about him and his 'boys' this season) and so in his mind, he had to get out there and throw a punch, lest his boys declare him 'soft'. But after he did it, he realized he was in the wrong AND that he was about to get his butt whipped, so he took off, hoping that Big Daddy Stern didn't see him sucker punch the guy.
Oops.