The Haymaker riot broke out in May, 1884 when a group of laborers struck the International Harvester plant over an eight-hour workday. During a clash with police, one striker was killed. Later, an anarchist rally was held to protest the killing. When the police showed up to break up the rally, someone threw a bomb, and killed a policeman. Several anarchists were tried for murder; four were hanged. All but one were German speaking; but most importantly, the one had a Knights of Labor Card on him. This event caused a tremendous backlash against labor organization, and the Knights pretty much faded as a result.
The prevailing fear among many Americans at the time was that labor organizers were primarily anarchists and communist radicals. The Haymarket riot did nothing for the image of organized labor, but rather reinforced the stereotype.