There probably wouldn't have been any trouble if the United fans hadn't of been looking for it .... nothing had apparently occured before their arrival.
The rags did a runner when the cops came calling , as you would expect , and obviously the City fans in the pub were trapped inside as the staff had previously attempted to close the doors/shutters .....
if it had of been three cops inside the pub who were attacked , every possible effort would have been made to find those responsible , no doubt about that .... but three (probably well-oiled) City fans are much easier targets than a bunch of running yobs , and especially if the bar staff failed to back up their story of what happened.