The police were too close when they approached, if they maintain a bit of a distance then it may not have kicked off or they would have been in a better position to control the outbreak.
I can imagine them thinking that as he had already been violent and his attention was elsewhere, they would take the opportunity to get hands on him from the start to control the arrest. Of course that approach assumes that once someone realises the police have got them they will show a degree of compliance, rather than go into full on berserker mode.