It is always hard to say what the "initial" cause is- the initial cause
is always caused by something else. I'd say in this case that the
initial cause was the positioning of Soviet nuclean weapons on Cuba,
which was a very tangeable threat to the US. The noticing of these
weapons by the US marked the start of the Crisis.
But you should
also see the positioning of the weapons on Cuba in a wider context of
the Cold war. Specifically, Cuba felt threatened due to the Bay of Pigs
Invasion in 1961 ( a year earlier) and this caused Cuba to invite
Soviet weapons as a protection.
