Ejector,
When you pull the triggers on a Dash and move the power levers back, you have discing mode which, at the forward limit of travel will give you quite a bit of forward thrust. At the rear limit of travel (reverse detent), you will get either zero thrust, or a very slight reverse (depending on rigging).
Discing covers a range of about 2-4" of power lever travel AFT of the flight idle stops before you hit the reverse detent.
Once you retard the power levers aft of the flight idle stops (in flight), all bets are off. It does not require the selection of reverse to lose control of propeller governing. The selection of discing alone will achieve that nicely, and long before the reverse detent is reached.
Not that I'm suggesting this is what happened in this instance.