PN, in March of 1991 two P-3's hit each other in mid air. (San Diego, CA, op area). One was coming on station and one was leaving. Our squadron had some tasking to support the SAR effort. One of our detachments was on a ship in that same exercise ... what little was found was pretty grim. It was a head on collision.
VPNAVY - VP-50 Memorial Page
(The best clue as to "why" based on info form our higher HQ was that there had been a miscommunication with one of the aircraft from the exercise controllers on ingress and egress altitudes ... )
(If you scroll down the Tracy Kreckman
at this link you'll get a hint of what someone saw when the collision happened).