A premature reserve opening definitely puts the "X" in XRW. These guys were in the middle of a sick XRW jump where everything was going according to plan (we’re going to skip over the part when they busted through the clouds, haha!). As the canopy pilot's feet pulled up on the wingsuiter's chest strap, it popped the reserve pin and the reserve pilot chute deployed! Fortunately, even without electronic communication equipment, the other wingsuiters in the flock were able to shout loud enough to convey that there was a problem. The wingsuiter with the premie managed to roll over before his reserve opened — he didn’t even have any line twists! Overall, this situation was handled very well and ended well.
These guys were incredibly forthcoming about what they believe happened here. In the details they provided, they explained that wingsuiter was flying a borrowed rig and it turned out that the reserve cable was about one inch too short. When the canopy pilot hooked his feet into the wingsuiter’s chest strap, it began to apply tension to that cable. When they started hitting a bit of turbulence the additional tension was enough to pop the reserve pin and fire the reserve pilot chute. No bueno.
The extra tension that is going to be applied to a rig is a foreseeable and obvious aspect of an XRW jump where a canopy pilot is hooking their feet into a chest strap. Knowing that, checking to make sure that the reserve cables are long enough to deal with that extra tension is a relatively easy additional gear check that can happen on the ground. This is actually something which jumpers who do a lot of hybrids check for because the tension seen in this video is effectively the same as what occurs on a hybrid when a jumper who goes into a stand and is holding onto the chest straps of jumpers who are on their bellies.