From a FFpoastboard member (Bex) here's what happened:

"It started because 2 idiots started smoking, another guy had a beef with this and made it known, banter ensued then some shoving and a marvellous uppercut from the non smoker to the smokers arse That's when it got nasty though, the smoker turned around and piled on the other guy getting him in the face a few times. "

It was at the Itunes festival!