A while back, an F-16 pilot inadvertently strafed a school in New York with his 20mm. On the F-16s control stick ,the trigger is used for more than one purpose. Depending on which menu you have pulled up, the function of the trigger changes. For example, in the New York situation, the pilot was trying to use his GPS waypoints, but had the wrong menu pulled up. So instead of getting his new waypoints, it fired the cannon. Maybe something like this caused the training bomb to be dropped.