To me this looks like a classic "Vmc" incident - Full power on one engine only and below the red line speed. That is what I'd put my money on. Why this happened is another matter......Anyone who as done a "Vmc demo" knows how quick the thing can roll on its back if not corrected and the only way to correct is to close the throttle.
I don't think this is asymmetric flap deployment for the reason that normally you wouldn't wait until you are over the airport for the final stage of flaps, and probably it would be more controllable with an approach speed.