Octane,
I should have added that to achieve negative torque, the engine must have failed or had its torque production reduced down and past zero.
There is speculation that the L engine of this King Air had its torque production reduced by the action of the power lever sliding back from the takeoff position.
Note that the prop RPM is varied by varying the blade angle. These blade angle changes directly increase or decrease the torque and can cause the torque to go negative, if it was originally a small, positive value.
In the crash of the Colgan Airlines Dash 8 Q400 in 2009, I believe that a causal factor was the sudden transition to negative torque that occurred when the pilots increased the prop RPM as part of their pre-landing procedure. I believe the torque was a small positive before this, but the torque decrease from changing the blade angles made it negative - thus precipitating a dramatic reduction in airspeed.