Of course, the ultimate cause of the 1984 Summer Olympics in Los Angeles was the recreation of the Olympic Games in 1896. But I do not imagine that this is what you are asking about.
The main reason why these games were important, historically, has to do with the Cold War. I would point to this as the "cause" of the games. The main fact about the LA Olympics was that the Eastern Bloc boycotted the games. This happened because of Cold War tensions. Specifically, it happened because of the Soviet invasion of Afghanistan in 1979 and the US boycott of the Moscow Olympics in 1980 in protest of that invasion. Because of the US boycott in 1980, the USSR and its allies boycotted the 1984 games.