What caused the Cuban missile crisis?

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.