There is a school of thought that hyper-aggressive policing can actually trigger violence in crowds (the Justice Department says this is one reason the Ferguson protests got out of hand). From what I saw on the ground, I'm now convinced this was part of it. No, this isn't blaming the police -- they're trying to keep everyone safe. But there are elements of crowd psychology that come into play here.
The most violent, and frightening, the RNC ever got was when these assholes showed up:
The police immediately formed a human wall between the God-Hates-Fags folks and the protesters, who took to screaming and cursing and almost seemed like they might start throwing punches. But when those same chucklefucks showed up at the DNC, the Philadelphia police had a very different response:
That's still a cop in the center, and there were still police around in case things went violent, but they didn't separate everyone. The protesters responded with more good-humored mockery, like that guy's Pokemon sign ("A Wild Bigot Appears!"), rather than screaming and shouting and waving fists.
Why would having a wall of police between them make people more aggressive? A few reasons. One, have you ever screamed at somebody who cut you off in traffic? Would you have done the same if you'd been standing next to them on the sidewalk, without two tons of steel protecting you?
Then there's just the general atmosphere it creates, sending the message that this is a place in which we are fully expecting unruly behavior. There's also a related phenomenon psychologists call the "weapons effect" -- the mere presence of weapons makes people more likely to engage in aggressive or violent behavior. Humans are social creatures, and we respond to the most minute cues from our fellow humans. It really doesn't take much.