This guy was helping a friend work on their sit fly. They break off, he tracks away nicely, waves off, pitches, and BOOM! You can actually see one of the broken lines fly past his face! He looks, knows what's up, and immediately chops. He doesn't have an RSL and his altimeter had rolled over on his wrist, so he pulls his reserve immediately as well. He thinks his arm caught one of the deploying risers, resulting in heavy line twists on the reserve, but he does a commendable job getting out of them and lands back at the DZ.
We really don't know why the lines broke, but — while this was a decently loaded canopy (a JFX 114 loaded at 2.3) — it doesn't look like a particularly hard opening. So we're guessing the lines were worn out and the jumper kept thinking he could get a few more jumps out of them before he replaced them. There's really no way to be sure but that's our guess and if that was the case, then this video is a great example about why you should replace worn lines.
You know how the canopy manufacturers, your coaches, riggers, and packers are always telling you to replace your lines? Yeah. You know where we're going with this one... it's really easy to keep thinking that you can get a few more jumps out of those raggedy frayed lines but there's a reason the previous owner gave you a discount on the canopy while telling you that you should probably change them out in fifty jumps or so.
IF this jumper was jumping on frayed out lines (again, we don't know for sure if that was the case) then he made a lot of good decisions after that one bad decision.