First of all their track was not Nadzab-MD direct, but rather via waypoint Yati (keeps you in the large markham valley). DME arrival into Madang requires a 2x profile due to the Finistere range. 2000fpm, 220-230KIAS and flight idle makes this easy. put the props up to 1050 if required for extra drag. 13000' at 24 miles is no big problem.
As the power levers are retarded towards flight idle, the prop governor hits the low pitch stops at 26 degrees blade angle. This area between 26-19 degrees (flight idle) is now flight beta where the HMU (hydromechanical fuel control unit) and ECU (electronic control unit) are controlling engine and prop rpm directly via fuel scheduling. The powerlevers are in direct control of blade angle in beta.
As the flightcrew actually set off the overspeed warning horn and consequently pulled powerlevers back to flight idle it seems possible that aerodynamic force caused the props to overspeed (the governor being taken out of the loop) and the HMU and ECU could not reduce fuel flow because the powerlevers are already at flight idle. This is a big design fault from Bombardier and they know it, but don't want to admit it. The FAA also knows it which is why they issued their beta-lockout protection AD dating back to 2006. Under the right (or wrong) circumstances this could happen again.
Hope this is clear.