The US invasion of Grenada from 1983 might have had more to do with what happened in the US and in other parts of the world than it had to do with what happened in Grenada. However, there were events that occurred in Grenada that were used as a justification for the invasion.
The main immediate cause of the invasion was the assassination of Prime Minister Maurice Bishop. Bishop had come to power in a coup in 1979. His government was socialist and moved to forge closer ties with other like-minded governments in the region, particularly that of Cuba under Fidel Castro. Under Bishop, the government was working to increase the size of the island’s airport. This was ostensibly done to allow larger planes to land, bringing more tourists, but the US was worried that it was meant to accommodate Soviet military planes. In 1983, Bishop was overthrown and murdered by people who felt that he was not sufficiently radical. The US, fearing that the new government would be even more pro-Cuba and pro-USSR, invaded. The US justified the invasion by saying that Americans on the island were in danger and by saying that there had been requests for help from a club of Caribbean nations and from the Governor-General of Grenada.
Many people would argue that the events on Grenada were not the real cause of the invasion. They would say the invasion happened because President Reagan wanted to achieve a victory over communism that would increase American morale after such things as the defeat in Vietnam, the Iran Hostage Crisis, and the bombing of the Marine barracks in Beirut. In this view, events on Grenada were simply a pretext. However, since your question specifies that you want to know what happened on Grenada to cause the invasion, the best answer is that the construction of the airport, the moves towards greater socialism, and the assassination of Maurice Bishop led to the invasion.