The inaction of the police in the incident involving Nick Watts is under a police investigation; and the action of the insulting and threatening demonstrators has been widely condemned by, among others, the BBC, Number 10 and Priti Patel. It is reported that the demonstration, attended by Piers Corbyn, was largely peaceful but obviously, not entirely.
Social Media plays a part these days but demonstrations that turn violent are not new. Either there are those intent on causing trouble within the groups or the groups take on a mob mentality. If alcohol has been consumed, it just oils the wheels of violent protest.