Why were there race riots?
You are speaking about the part of Middlesex when Cal and his family get caught up in the race riots of 1967 in Detroit, Michigan. This is important because Cal's family's home is invaded because President Johnson ordered in federal troops to control the race riots. In reality, though, your question has very little to do with the novel. Researching the riots in Detroit in the year of 1967 will tell you the answer.
in short, there were race riots because police in Detroit invaded an unlicensed bar called The Blind Pig. The resulting altercation lasted five days and became the most deadly example of riots in the history of the United States. The fighting and the death toll escalated to such an extent that President Johnson had to send in his federal troops. Many feel that this riot was a direct result of the segregation and discrimination the blacks in Detroit received because of the influx of African Americans to Detroit during the Great Migration. Racism and discrimination continued. Many feel that this discrimination still exists today.
In conclusion, the race riots happened because of the racial discrimination that African Americans felt in Detroit in 1967. This directly affects the characters in the book when police invade the home of Cal and his family.