I haven't heard a likely explanation yet, I don't believe in engine failure, because that should be easy (Yes, famous last words) to handle: Just settle down somewhere around, they were still in the vicinity of the runway so lots of good landing spots.
More likely, bad CG or rapidly shifting CG? Mechanical error, of course. But this was an experienced pilot, and a "routine" flight, how "easy" would it be to overwhelm a plane like this with ill positioned jumpers? I get the feeling the airplanes are really cramped, and if so the CG should sort itself out naturally.