From the DC Protest stream last night:

Watch how the crowd goes from chill and peaceful to loud and crashing on the fences the instant CNN starts filming.

As far as I can tell, the man yelling "Quit your job!" might be the signal. He's a fresh voice in an otherwise exhausted and incredibly hoarse crowd, and his chanting began as soon as CNN organized to start their stream.

The reporter did a dry run–and was very annoyed by the presence of the streamer filming. At the same time, a potential antifa goon - dressed in black with gloves and his face completely hidden save for his eyes - starts interrogating the streamer. Note how he asks "Who are you with?" but then follows with identifying him as either a vocal supporter of Yang or on Yang's campaign. He's also been watched for the past five days with the "protester" noting that he's "changed his hat."

The CNN reporter then does a second take as the crowd gets a little rowdier, but it isn't enough. On their third take is when the instigators head to the fences and the chanting REALLY starts, despite the other protesters yelling, slapping, and pulling the agitators away from the fence.

Until that point, the remaining protesters had been relatively quiet, chill, and hadn't bothered with the fence. Then, when CNN needed a clip, they coordinated and got the crowd intentionally riled up. Eventually, the police fired off tear gas and pepper spray to back them up.

https://www.youtube.com/watch?v=N1Z7Wp5hZR0&feature=youtu.be&t=24633