I'v been wondering about this tumble since it was first posted. It seems to be the first, or first ever reported anyhow, so very rare. This airplane has balast in the tail to compensate for the extra weight in the nose. In other words, weight at the extreem ends of the longitudinal axis.
Weight at the extremes alters the moment of enertia. The moment for weight and balance purposes is calculated by multiplying arm times weight. Not readily apparant is that moment of enertia is arm squared times weight.
A small weight with a long arm can significantly alter the enertia.
When the flap handle is pulled up the nose pitches down which started the weights in the nose and the tail to swing around the center of gravity. Could the enertia been too much for the control surface...stab trim and elevator...to overcome?
The flap overspeed theory makes good sense to me. Maybe the extra weight at the extreme ends of the fuselage compounded the effect in this particular airplane???