This skydive student's 4th static line jump escalated quickly with a premature reserve parachute opening that ripped him off the plane step! While climbing out of the small aircraft, the reserve flap came undone and exposed the reserve pin. He unknowingly dragged the top of the container across the plane's door handle above him, resulting in the reserve pin getting dislodged which caused the premature opening. The student was ripped off the plane step by the reserve, barely missed the tail of the aircraft, and — of course — his static line also deployed his main canopy, resulting in a two-out. He cutaway the main and landed safely on his reserve.
The jumper appears to have been sitting up against the instrument panel, may have rubbed up against the pilot's seat while getting up, and certainly rubbed up against the door and the door handle as he got out onto the step. Any one of these moments could have resulted in a premature deployment. It's actually fortunate that it wasn't until he was outside the aircraft that this jumper finally snagged something so roughly that his reserve deployed. Had that happened while he was still in the plane, it could have resulted in him getting yanked out, and possibly sustaining a fatal impact on the inside of the doorway as he got pulled out.
If you watch closely, as soon as the student begins exiting the plane, the flap protecting his reserve pin just blows away, leaving the pin unprotected. This should not be feasible with properly maintained gear. The possibility exists that the student rubbed the flap and displaced it while getting up, but the video suggests that this may have been older gear that needed some love from a rigger.
It's important for skydivers to know how to exit any given aircraft and to know about potential snag points that could catch your equipment. You'll often see experienced jumpers reaching back to check their main flap and their pilot chute anytime they move around the plane and then again one last time right before the door is opened. What they're actually doing is checking to make sure that they didn't inadvertently catch their container on something that could result in an unsafe situation. Further, once you're aware of those snag points, it's important to remember not to rub up against them!
The flaps protecting your reserve pin and main pin start to wear out over time and they lose the stiffness required to remain firmly seated. They need to be kept in good repair and, if they're so weak that nothing more than the wind can uncover your pins, it's definitely time to have those flaps replaced.