Here is a clue:
http://www.airdisaster.com/reports/ntsb/AAR94-06.pdf
Not saying this is what happened here, but way back in '94 a Saab 340, for some reason (read the report) ended up with both engines in reverse range in flight. Both props ran away; one going to 142%, the other to 158%. Both engines flamed out, but they were able to get the engines running again.
Didn't help, though. In a free turbine engine there is no direct mechanical connection between the compressor (gas generator) section and the power section (which drives the propeller). While the overspeed event did not damage the compressor sections, it destroyed the power turbines in both engines. So even though both were successfully restarted, with the power turbine blown out the back like a mouthful of sunflower seed husks, there was no way to deliver power to the propellers.
Unlike the accident in PNG, there were no serious injuries among the pax and crew of the Saab. And again, I am not saying this is what happened. but there are similarities between the preliminary report on the PNG crash and the NTSB report referenced above.