Listening live to the scanner in NY something sounds like a planned operation - they were setting fires and doing things to get more and more cops away from police HQ (if that is what they were using as a dispatch point) then the mob dispersed and a different mob descended on HQ
Background on that, btw is that they were setting fires at University and 13th Street and moved downtown; over time the cops dispatched more and more officers to... look at them I guess