Some of these were ideological; Mobutu was a staunch Cold War ally of the US and was highly critical of communism.  His experience with Soviet-supported Katangese rebels in the late 1960s helped establish these views and when this group began to fight on the side of the MPLA in 1975, it only increased the animosity between Mobutu and the MPLA.  Other aspects of the MPLA and Mobutu's ideologies also clashed.