I was under the impression that Kayal's leg was up in the air as Barton had kicked him twice in the back of the legs, thus tripping him up. That was the foul that the Ref gave, not the stamp.
Dunk had issues with Barnes before Barnes even got on the pitch. Dunk wandered over to get a drink right next to Barnes when he was waiting to come on, had a few words then tapped him the side of the face. It didn't appear to be particularly friendly.