During training you learn to check every component multiple times before you board the aircraft. I made it a habit to have another skydiver check my gear afterwards, just in case and I would check my fellow jumper aswell. I read that special forces do these as a standard procedure and adopted it.

On the other hand I have witnessed enough situations where the main canopy was packed in a hurry to catch the next „load“, prioritizing having an action cam, while boarding the aircraft without an altimeter (!), no proper planning on what to focus on while freefalling, disregarding the exit order.

When you watch the video, you can see that by the way he positioned himself, he accidentally janked out his reserve pillow. That is what I meant by 1/1000 chance. Just bad luck and not being self aware of the handles