Slid off the runway? It is a bit difficult to tell with the video but does it look like they taxied off or slid off? Not sure if the video is in real time, so does it look like they were taxiing a bit fast?
It was a long time ago but once upon a time, I did head toward a dark area while aiming for a couple of further away taxi lights and only intervention from the instructor saved my ass. Of course most experienced guys know to follow the centerline of the taxiway but I'm sure the investigators will at least ask if that scenario was a possible cause among the many other possible causes.