What is the cause of the republicans fighting?

To defeat the facist army of Francisco Franco.

Well considering that the 'Republic' was already proclaimed, the farce of the actual war and the reasons for fighting were pure political ideology, loyalist Leftist Loyalist Communists against Fascist Monarchists.

So we have Communist Republicans and Fascist Monarchists, then the Democratic countries side with the Communists and so do the Soviet Communists, against Germany and Italy aiding the Monarchists....weird considering they both over threw their own monarchists.

So in fact the 'cause' was power and money, same as it always is.

Civil war is just war, there is no nobility.