In waves of civil unrest—including recent examples such as the Black Lives Matter movement—structural injustices and histories of antagonism explain which cities riot, but not when they riot. Police preemptive mobilization may draw a crowd, inadvertently expanding the numbers on the streets. User rustoo, in the Reddit Science subreddit, 15 Oct 2021
Data from spread of rioting to one English city showed rioting in other cities led to police vigilance, resulting in police mobilization, creating a large gathering in a community with a history of hostile relations with police, and police attempt to disperse the crowd lead to collective conflict. User Psychnews, in the Reddit Science subreddit, 14 Oct 2021