What were the causes of the civil war in china?

There were many internal disputes in China after the 1911 revolution, most known is what I believe the one you are referring to, which is when Mao came to power.

Following the end of the Second World War, many internal conflicts in China was set aside to unite against the common enemy; Empire of Japan. When Japan was defeated by the US, China regained their territory and subsequently the conflicts reemerged. The two great forces seeking power were the Nationalists and the Communists. Despite the fact that Nationalists had prior to the war been massively superior and were supported internationally, they had had such losses during WW2 that the Communist Forces managed to win power and Mao gained power there in 1948.