This was discussed a couple of months ago. However, a small contributor is the reduction in the downwash on the horizontal tail surface(s); but the big one is the reduction in flapback. The stick position is forward in the cruise to overcome it; it is proportional to collective pitch; hence when you reduce collective, you don't need so much forward cyclic. If you leave the stick where it is, the nose will go down, so you have to pull it back.
Think I've got that right.