The correct answer is "pressure from anti-communist republicans."
After World War II, the US and Soviet Union emerged as global superpowers. Their difference in political ideology lead to a battle for global power and influence. This battle caused for the US to get rid of any possible communist influences or Soviet spies within the federal government. This pressure, put on by Republicans who felt that Democrats were "soft" about getting rid of communism, lead Harry Truman to put into place the loyalty program (also known as Executive Order 9835).