WHAT REALLY PUZZLES ME:
According NTSB the shakeup occured when the flaps where in transit to 15°.
I think that minimum speeds reduce faster during flap transit, than the airplane decelerates.
Ergo : if it was a normal wingstall it should have occured earlier, when flaps where at 5°.
Maybe adding full power at such a low speed was the factor that destabilized the A/C.
Flying at low speed (near idle) and then adding full Power feels quite "brutal" in the Q400. Maybe adding the power was the factor which brought the A/C into a kind of "dynamic" stall.