I happened to be in downtown Detroit in 1968 when the Detroit Tigers, playing in the old Tiger Stadium very near downtown, won the World Series just at the close of the business day. The downtown area of the city erupted into a gigantic party involving thousands of people and lasting for about eight hours. Collective behavior needs both an social environmental context and a stimulus or trigger. Researchers studying the social context of lynchings of Afro-Americans in the South found that lynchings were more common when the price of cotton was low, putting strains on the economy, than when they were when prices were high. Triggers or stimuli for the expression of collective behavior are the events that create the heightened emotion that leads people to get involved in the collective action.
Rumors often play a part in the triggering mechanism.
The social context involved the Vietnam War and claims that poor Blacks were disproportionately being drafted because better off whites could go to college and get deferments. It also involved unfulfilled but rising expectations for lower Black unemployment because of the civil rights laws passed a few years earlier. In Detroit, the trigger mechanism was a police raid on a motel to apprehend suspects in a crime. Immediately, rumors began to spread about police over-use of force and about the murder of one or more suspects by the police during the raid. As it happened, it was hot and large number of people were out of their houses and apartments on the streets and on porches and balconies, which assisted the spread of rumors.