This depends on how you mean the question.
The most immediate cause of the Cuban Missile Crisis was the fact that US U-2 spy planes detected Soviet missiles in Cuba. The US felt that this was too much of a threat to US soil and therefore Pres. Kennedy started to take actions to force the Soviets to remove the missiles.
If you take another step back, the cause of the crisis was the fact that the Soviet Union did not have missiles that could hit the US mainland while the United States did have missiles that could hit the Soviet Union. This imbalance made the Soviets uneasy and led them to try to even it out by putting missiles in Cuba.