From what I've read elsewhere, the off duty CHiP was rescuing the girl whose phone had been stolen. She tried to get it back from the thief and the group began to beat her. When the CHiP intervened, they transferred their attack to him.
These punks are lucky to have survived the incident. Even though he was off duty, the CHiP retained his police powers. Eight on one and choking the CHiP out made this an attack with deadly force that justified defense with deadly force. Yes, there would have been screams of (false) outrage by the usual suspects and maybe even another riot.