Burst of violence can occur anywhere. Like a burst of rain, or wind shear, it just materializes out of thin air, and rains down knives and bullets.
Last time I was in DC, 2013, wife and I were eating lunch at a restaurant just down the street from the Capitol One Arena.
A beverage delivery truck pulls up across the street and suddenly 20 or so “youts” rush in and strip the thing clean. By the time the cops arrived, they were all gone. Waiter tells me they coordinate on their Obamaphones.