Mob stupidity. Local team wins (insert pro sports league here) championship. Riots and burns own city down. Personally, I'm convinced there are pro's who stir this stuff up so they can commit crimes under the cover of the other rioters.

To make this a sort of on topic post, back in the bad old days of industrialization in the late 1800s, strikes often had "useful idiots" to get the desired result, a violent crackdown by the authorities. Thus, the general public would sympathize with the union members.

This approach backfired during the "Great Upheaval" in 1877. Over 1400 strikes took place that year and most turned violent. The impact of this turned the public against the unions to a degree, the opposite of the desired effect. The unions had many legitimate grievances such as unsafe working conditions, pay cuts without corresponding reductions in rent in the company town, etc.

The Great Strike which was one of the strikes that started the Great Upheaval took place on the B&O after the company cut wages three times in less than a year in 1877. Once these things turn ugly its like tossing a match on an open bucket of gasoline.