Ouch! The torque came out...
(Assuming the initial condition to be trimmed in pitch, roll and yaw and flying straight)
Engine torque acts along the prop shaft. This means it will attempt to roll you around the prop shaft on a power change – and that’s it. If you try to counter this roll tendency with aileron, this will create adverse yaw, the direction of which is left as an exercise for the reader.
Gyroscopic precession will not create yaw until there’s a pitch change.
P-factor will be there, unless the flight path vector is parallel to the prop shaft. The direction of the yaw will depend upon the angle of attack.
Now, the likely culprit: Helical prop wash. The slipstream from the propeller coils its way down the fuselage, striking the fin from one side, creating a side force on the fin. You trim to fly straight, then reduce power... and the strength of the slipstream is reduced. This side force reduces with it and you get... yaw.
Regards,
Fred