In the early 20th century, there was an unwritten rule that one was not supposed to wear straw hats past September 15. If someone was seen wearing a straw hat, they were, at minimum, subjecting themselves to ridicule, and it was a tradition for youths to knock straw hats off of wearers’ heads and stomp on them.

The riot itself began on September 13 of 1922, two days before the supposed unspoken date, when a group of youths decided to get an early jump on the tradition. This group began in the former “Mulberry Bend” area of Manhattan by removing and stomping hats worn by factory workers who were employed in the area. The more innocuous stomping turned into a brawl when the youths tried to stomp a group of dock workers’ hats, and the dock workers fought back.

Although the initial brawl was broken up by police, the fights continued to escalate the next evening. Gangs of teenagers prowled the streets wielding large sticks, sometimes with a nail driven through the top, looking for pedestrians wearing straw hats and beating those who resisted.

Via Wikipedia.