What led to a resurgence of the Klu Klux Klan in the 1920's?
Many of the "new immigrants" of 1880 - 1920 came from southern and eastern Europe, countries with largely Catholic populations. Catholic immigrants had been discriminated against since the Irish immigration waves of the 1840s, so it's no surprise that more people joined the Ku Klux Klan when it became anti-Catholic in the 1920's.
At that time, almost one out of three Americans was either a first or second generation immigrant, and many people felt that their traditional culture and way of life was under attack. Along with Catholics came the first large scale Jewish immigration into the cities and garment districts of Boston and New York, and anti-Semitism became part of the Klan's mantra also.
During the 1920s, the US was undergoing a sort of culture war between traditional people, often rural, and people with newer values, most of whom lived in the cities.
This change had come on the heels of a whole lot of immigration in the 40 or so previous years. Many of these immigrants had settled in the cities.
The KKK of the 1920s was much more anti-immigrant and anti-new ideas than it was anti-black. One of its slogans was "100% Americanism." It stood for the traditional values that it thought the city people were destroying.
So, the KKK reemerged as part of a backlash against the changing values of the 1920s.