The guy was bar hopping with his friends. They already had a snoot full and got stopped at this one bar by the bouncer. Johnson has some words with the bouncer and the bouncer informs ABC cops this guy is drunk, underage and tried to get into the bar. ABC takes him over to the side, quickly see he's drunk - they get his id and find out he's under age. Johnson get's mouthy and abusive. Cops decide they're going to charge him and that he's too drunk to send home so they attempt to cuff him. He resist. They have to wrestle the guy to the ground where he is injured during the scuffle and bleeds. Johnson yells "racist" because even in his drunk state he knows someone would be getting it on camera.
Now... I might be 100% wrong and the cops may have beat the **** out of him but until such evidence is shown, that's what I think happened.