Mere curious speculation:
I was thinking maybe an engine fire might have heated the wing spar to the point of failure? Didnt such a failure down a turboprop in Canada a few years back, after a landing gear fire in the gear well started from a brake dragging on T/O?
That was a Metro:
http://www.tsb.gc.ca/en/reports/air/...7/a98q0087.asp and (correct me if I'm wrong) there are no hydraulics in a Mallard's wing, the wheels retracting into the fuselage which appears undamaged in the video.