It's because he made menacing faces and was breathing heavy.
They'll probably do some goofy Punk/Show vs Rock/Cena match at some point. Or better yet, the 'WWE special' where people who hate each other are forced to team up, so it'll be Punk/Rock vs Cena/Show. WHAT WILL HAPPEN??!! CAN THEY CO-EXIST!!!!!!?????
Punk was due for a heel turn, but I agree, from a character standpoint its hard to blame him for leveling The Rock. If this was 1996 ECW all four of these guys would be different shades of gray, with Cena being almost all white and Show being almost black.