Shadow,
Nothing to do with props,
As the airplane pitched nose up and the AOA increased through 5 degrees, the airflow in the area of the right aileron began to separate from the wing upper surface because of the ice ridge. As the AOA continued to increase, the airflow separation in the area of the right aileron also increased, causing a reversal of the right aileron hinge moment characteristics. Although the right aileron hinge moment reversal caused the ailerons to deflect rapidly to a right-wing-down (RWD) position, the AOA was not sufficient to activate the stall warning system prior to the aileron deflection. The autopilot could not control the aileron deflection rate, which exceeded that allowed by the autopilot so the autopilot disconnected.
Within 0.25 seconds of the autopilot disconnection, the ailerons fully deflected to the RWD position79 and the airplane rolled rapidly to the right until reaching 77 degrees RWD. An immediate nose down elevator deflection reduced the AOA; and the ailerons were deflected LWD by the flightcrew to counter the right roll. The airplane began to roll back towards a wings-level-attitude. The crew then applied 2 to 3 degrees of left rudder and nose-up elevator. The flightcrew's aileron and rudder control inputs reduced the bank angle to 55 degrees RWD. However, as the AOA increased to more than 5 degrees, the airflow over the right aileron separated again, resulting in a second aileron hinge moment reversal and rapid RWD aileron deflection.
This is directly from the NTSB report, it was a flaw in the aircraft design. The flow separation actually commanded the aileron movement not just a simple wing drop stall. The deflection occured at around 180KIAS and only 5 degrees AoA.
The sad part about the whole scenario is that the aircraft had demonstrated aileron hinge reversal on clean airframes during certification at low AOA and they had adjusted the airflow in the area to increase the angle at which it happened. After the first icing related ATR-42 accident in 1987 they knew that it was most probably ice induced aileron reversal and put out a training package and added more VG's to fix the problem. They did not inform any operators or pilots of the findings. The training package did not cover sudden full aileron control deflection with up to 60lbs of force during apparently normal flight (which was what happened in both accidents).
There was a massive lawsuit against ATR because of this.