Police are on a common radio channel and coordinate their actions. Protesters aren't. If one protester takes violent action, most protesters may be unaware but all police will be. What you see as an unprovoked, violent response may actually have been provoked by something you simply weren't close enough to witness.
This explanation is pure speculation. However, if true, it does prompt us to ask if police can find ways to localize their responses better. i.e. If violent response is necessary in a specific location, is there a way to make police relatively far from that location aware of it without signalling imminent personal danger to them, thereby priming them to over-respond to whatever is happening at their location?
e.g. Instead of just hearing "They've pulled out knives!" over the radio, they might have a corresponding heads-up display showing where that message came from and where they themselves are in relation to that.