what causes the cuban embargo?

In addition to what RT said, it was also the Russian sovereignty with Fidel and Cuba that scared the crap out of the US government since this was during the Cold War.

When Castro took control, he nationalized evweryhing, basically seizing and taking everyone's private property. The refugees fled to the US. That pretty much pissed off the US government, so they put the embargo in place to try to make the people drive Castro out.

In November 1991, the Cuban ambassador, Ricardo Alarcon, in a speech to the UN General Assembly, cited 27 recent cases of trade contracts interrupted by US pressure. The British journal Cuba Business claimed that British Petroleum was seemingly dissuaded by US authorities from investing in offshore oil exploration in Cuba despite being initially keenly interested. The Petroleum economist claimed, in September 1992, that the US State Department vigorously discouraged firms like Royal Dutch Shell and Clyde Petroleum from investing in Cuba. However, this pressure did not work in all cases. According to the Mexican Newspaper El Financiero, the US ambassador to Mexico, John Negroponte travelled to meet two Mexican business men who had signed a textile deal with Cuba on October 17, 1992. Despite the representation, the deal went ahead and was eventually worth $500 million in foreign capital. All of this happened before the signing of the Cuban Democracy Act.