Erm...the same reason as it goes left!?
It's the opposite effect surely. Increase power > increase torque > nose wants to slide left. Decrease power > decrease torque > less tendency to turn left therefore, all other things being equal (inc. control deflections administered by the pilot), nose slides right i.e. to an equilibrium position. The aircraft is designed as inherently stable and will want to return to its most stable position readily in this situation.
That's my take on things. I stand to be corrected though.
Regards,
V1R
P.S. I assume in some aircraft this sliding to the right does indeed happen!