I do not know how F-18 controls work but from what I understand lots of jet controls include the equivalent of a "safety" that can be used to prevent the weapon from being launched. Maybe the pilot thought he had it engaged?

The secondary thing here I've realized is that the missiles in question must not have been using active homing. If they were then the pilots of the US aircraft would have taken evasive action as soon as their radar warning receiver lit up.