I wonder if setting the fire and breaking things was done by the Chinese government to discredit the protesters. Like the Reichstag fire by Hitler supporters.
It has been suspected that there are police doing undercover work as a protester in attempts to make the protesters look bad. However, it is well known that protesters do set fire to items during protests.
Also, during the weekend for the first time a car was set on fire. This, and the fire at the subway entrance, are an escalation of events. Who actually set them? My guess it was some protesters.