The turbine engine "model" that is used within Microsoft Flight Simulator is flawed in many respects. I can't speak for FS-X but know FS2004 (ACOF) very well.
On a real PT6 engine, the fuel flow does NOT change when the prop RPM is reduced.
The torque certainly increases, however. Victor India and Captahab are correct about the relationship between power, torque and RPM.
Note that it is the torque increase that causes the prop RPM to reduce.
The "first" thing to happen - so as to speak - is the torque increase when pulling the prop levers back. That torque increase "drives" the shaft RPM down to the lower value.
This all appears to happen simultaneously, of course.
Oh, and to answer your question:
The torque increases when you move the lever to effect a prop RPM decrease because the prop governor moves the blades to a more coarse (higher angle of attack) angle. The blades are thus taking a "bigger bite" of the air - and imposing more of a load on the drive shaft (exerting a greater torque, in other words).
Note that the torque being exerted on the engine by the prop is equal and opposite to the torque being exerted on the prop by the engine (during the theoretical equilibrium conditions, of course - which is about 99% of the time).
This is all true for the piston engine/CSU combination as well.