Severe Weather/Fire, Power grid failure and Virus/Super bug would cause resources to be brought in to deal with the issue. Lots of folks injured and dying causes those remaining/surviving to join together.

Economic Collapse on the other hand is a man-made issue that can drive the have's and have-not's to collide. Interesting times ahead.

If a Flash Mob/Terrorism resulted, it would be a result/symptom of the scenario and not a trigger.