Right SpyderMike, in the slomo shots you can see the wings flutter and debris being shed as the plane begins its downward plunge. Similar effect to that which brings bridges down, ie vortex shedding in a strong wind reinforcing resonant vibration. In the case of a plane, there are two main elements, 1 the torsional flexibility (natural frequency in torsion) of the wing, and 2 the position of the wing c of g and centre of lift with respect to the axis of the torsion. For complete stability the c of g and c of l need to be behind the axis of torsion. Strange to think that a little more weight along the trailing edge could have prevented this.
