This group was launching a linked four-way belly exit with outside video. However, seconds after they exit, one of the skydivers had a premature opening that ripped him out of the group! (Seriously though, they had some pretty solid grips on each other because you can see him quite literally get pulled out of the formation.)
There's no video from inside the plane, however it doesn't look like the jumper brushed against the door, and while the jumper had relatively old gear, we were told that on the ground the elasticity of the BOC was checked and it was fine. That means that the pilot chute was sneaking out before these jumpers exited and nobody noticed.
If you've ever jumped with folks who have a wall — or a drawer — filled with medals from winning nationals or worlds, you may notice how they almost look like they have a nervous tic from how often they check their pilot chute. You're getting on the plane, and they're touching the small of their back. The door is about to open to get some air, and they're touching the small of their back. They move around a little trying to get more comfortable, and they're touching the small of their back.
In short, pretty much anytime there's a chance that their pilot chute could have gotten dislodged, or anytime there's a chance for it to catch air, they're double checking to make sure that thing is still safely nestled in their BOC. It's a great habit to develop and once it becomes second nature you can be pretty sure that this type of incident won't happen to you.