Wikipedia:

The founding in 1915 of a second distinct group using the same name was inspired by the newfound power of the modern mass media, via the film The Birth of a Nation and inflammatory anti-Semitic newspaper accounts surrounding the trial and lynching of accused murderer Leo Frank. The second KKK was a formal membership organization, with a national and state structure, that paid thousands of men to organize local chapters all over the country. Millions joined, and at its peak in the 1920s the organization included about 15% of the nation's eligible population.[1] The second KKK typically preached racism, anti-Catholicism, nativism, and anti-Semitism, and some local groups took part in lynchings and other violent activities. Its popularity fell during the Great Depression, and membership fell further during World War II, due to scandals resulting from prominent members' crimes and support of the Nazis.

Apparently about 1 in 6 people were in the Klan at that time. So your great grand dad wasn't alone in checking them out.