If the propellers were placed into DISC (retarded below the flight idle indent) it is possible the aerodynamic forces of a rapid descent could cause the propellers to overspeed. In normal flight configuration the DHC-8 has an automatic safety feature that will drive the propeller to coarse pitch if it exceeds 1208 PRPM. This is driven by engine oil pressure pushing on either side of a piston and tachometer. In the oil is lost the propeller will lock its pitch.
Bombardier already has issued warnings about reducing the power levers below flight idle and in US air carriers a placard is mandatory stating this prohibition.
As a manager at another DHC-8 operator in the Pacific I will be interested in seeing how this investigation plays out.