Think of it this way:
In forward flight (airspeed across disk), there is dissymmetry of lift, which is being countered by decreasing the pitch on the advancing blade and increasing the pitch on the retreating blade - forward cyclic essentially controls the blade's desire to flap up on the one side and down on the other.
If the collective is raised to maximum allowable power, there is more lift, therefore more dissymmetry. With collective lowered to minimum, there is less lift, so less dissymmetry.
If you leave a high-lift flapping correction (forward cyclic) when reducing power, you will cause "flapping" the other way, so the nose will want to pitch down.