How did union strikes contribute to rising racial tensions in the 1920s?

Strike breakers--with the Great Migration many blacks were living in the North. When strikes occurred, blacks would be hired in the white workers place. 

Unions and communism--unions were often associated with communism. Being there was a Red Scare in the early 1920s, unions caused nativist sentiment to rise as well as anti-communist rhetoric. 

Both of these reasons gave way to a large renewal of the KKK which combined racism towards blacks and a white nationalist movement.