Well, there are many reasons, which is a mix-bag of ideology clashes, historical events, etc. And is kinda necessary to have a big picture of the spanish politics not only before the civil war, even before the second republic took into place. But to make it easier, I'm going to talk about the more recent events that led to those changes. The PCE was created in inspiration of the soviet revolution and model, and like the changes of the soviet system, it affected the communist party of Spain. In the late 1920s the PCE would start to get heavily influenced by Stalin's ideas (and the idea of socialism-in-one-state would be present among them), this would led dissatisfaction with the more traditional factions of the PCE and, of course, the trotskyists. In the early 1930s these factions would create their own parties, the ICE (Izquierda Communista de España) in 1932 and the POUM (Partido Obrero de Unificación Marxista) in 1935, this last one would absorb the ICE. During the spanish civil war, the PCE and the POUM would be on the same side during the first year, but that would change when the anarcho-syndicalists would start to revolt in 1937, which would make the POUM to switch sides with the CNT-FAI as they belived it was more faithful to the marxist ideals. The PCE would remain faithful to the goverment mainly because of their ideal of supporting the "struggle against imperialism" without caring the ideology of whom are they supporting. After the fall of the republic, many communists, formerly from the PCE and other small marxist organizations would later start organizing into militias resisting Franco's regime. But after the sino-soviet split, the PCE would take a pro-mao turn, kinda returning to some orthodox territory in terms of ideology.