I agree - people are talking about the fear that they could have grabbed the guns, and now they could be motivated by explosives or terrorism.
In that case - why the quick approach and grabbing someone - without knowing who else was involved.
People are talking about reflex actions - you are in a queue and you see 3 people suddenly grab your brother. You might not even recognise they are police, let alone armed police. You just see your brother being grabbed. In this case, he reacted by fighting quite viciously.
With hindsight - I wonder if the police would have approached it differently.
I also wonder if - within the time it took for it to kick off - if they even realised it was armed police who were trying to arrest him.