Issues of racial inequality had been simmering in the United States for a number of years. In the late 60s, despite requests for peaceful protests from Martin Luther King Jr. and other leaders in the black community, frustration boiled over into violence.
It would be naïve and inaccurate to say that people rioted in Newark, NJ and Detroit, MI because they were hot. But the long, hot summer of 67 certainly didn’t help.