If they forgot to pull the arming pins, the bomb wouldn't have dropped. Those pins controll/block jettison of munitions.
The arming wire controlls the bomb arming. You energize a solenoid that his holding a ring that pulls the arming wire out of the bomb as it drops. If that solenoid doesn't energize, the ring and wire drops with the bomb.
All-in-all... someone got lucky. I wonder if there will be any "those damn reckless dutch pilot..." threads that start because of this.
MiniD