I think it was more complicated than either nationalist agendas or spreading communism. In order to understand the origins of a revolution you really have to understand the context and condition the country was in that lead to the uprisings. In Cuba the revolution was motivated by a bunch of things that made it popular among the people. Essentially the people were dissatisfied with the government due to the gap between rich and poor, working conditions, corruption and foreign involvement in Cuba. During the 1950s Cubas population was on the rise and so was unemployment. A 1956-7 study showed that the 34% of the population who were rural workers received only 10% of the national income and 80% of these workers only had access to medical care through paying for it (Farber, 2006, p. 21). Combine this with a large economic gap between rich and poor and the Castro became pretty popular “In Cuba, he galvanized the popular masses, whose standard of living was far below that of the rich population in Havana.” (Divney, 1999, p. 1).
The involvement of the United States in Cuba's economic situation and politics, which had been going on since they helped the Cubans get rid of the Spanish in the Cuban war of independence (1895–1898), also contributed to the peoples dissatisfaction with the government and certainly to Castro's motivation for leading the revolution (Fidel Castro: My Life: A Spoken Autobiography). The Platt amendment 1901, gave America political and economic influence over Cuba which America used to get cheap sugar. They were able to fix the price of sugar taking money away from the country and the government passed laws making it difficult for Cubans to start up small sugar mills. All the bigger sugar mills were owned by American companies. Castro was against all of that and so he gained further popularity. strangely enough the Americans didn't see Castro as a strong threat to their interests they “could not imagine a successor regime breaking the [economic] relationship.” (Benjamin, 1990, p. 148). They didn't really like the current government and its dictator Batista at that point any more either because he had grown ineffective so they stopped supplying Batista with arms by March 1958. Essentially the reason why the revolution happened was because the people were sick of their dictator and the Americans. Of course communism was important to Castro and to the other revolutionaries that he brought with him from mexico but if the Cuban people had been happy with their government his revolution would not of had support and would probably not have been successful. I am sorry for the messy referencing. If you are interested you could read the book Fidel Castro: My Life: A Spoken Autobiography. It explains his motivations well although you have to keep in mind that he is talking about him self and so is bound to be biased in his own favour but it is a good primary source.

Benjamin, J. R. (1990). The United States and the Origins of the Cuban Revolution. New Jersey: Princeton University Press. Divney, J. M. (1999) Victory for Some, Defeat for Others: Understanding the Insurrection and Seizure of Power, 1952-1959. Farber, S. (2006). Origins of the Cuban Revolution Reconsidered. Chapel Hill: The university of North Carolina press.