This skydiver had an unexpected premature opening on his reserve parachute while sit flying at 10,000 feet (I suppose all premature openings are “unexpected,” right?). None of his handles were pulled, cable lengths were fine and the reserve closing loop was intact (no AAD misfire).
However, after inspecting the gear on the ground, it turns out the reserve closing loop was 12mm too long (Aerodyne recommends 110-123mm, this loop was 137mm). The long closing loop combined with rubbing the back of his rig while sitting in the rear bulkhead of the plane was most likely the cause of this premature opening.
Repeat after me: gear checks, gear checks, gear checks. Alright, good talk. See you out there 😉