Although a 'beta lockout' mechanism did exist for DHC-8-100,-200, and -300 series aircraft which prevented the propellers from going into reverse even if the power levers were moved into the beta range during flight, this mechanism was only required by regulation to be installed in DHC-8 aircraft operating in the USA. It was not required to be fitted to DHC-8 aircraft in Papua New Guinea, and it was not fitted to MCJ.
and then,
If a beta lockout mechanism had been installed on the aircraft, the double propeller overspeed would not have occurred when the power levers were moved below the flight idle range and in the ground beta range during flight. Installation of this mechanism is now mandatory on DHC-8 aircraft worldwide by 19 June 2016.
Fairly significant factor imo. Neither the Cpt or F/O had trained for an overspeed in the sim. He must have given the levers a fair pull to sail past the detente without noticing, but you would think ham fistedness shouldn't be able to drive you into ground beta in flight..
I bet they still haven't fitted the lock-outs yet though.