Answer
Zhu's designation as a "rightist" and subsequent expulsion from the Chinese Communist Party (CCP) occurred during the Anti-Rightist Campaign of 1957. This campaign targeted intellectuals and others deemed critical of the party and Mao Zedong's leadership. Zhu was among those labeled as "rightists" due to his critical views and outspokenness, leading to his expulsion from the CCP and subsequent political persecution.