Some is necessary, Lucky, to maintain fore-aft balance when bending the knees. Since our ski boots essentially prevent us from "breaking" at the ankles, or from lifting our heels--two movements we make OFF skis for fore-aft balance when flexing down--we must compensate by increasing the movement forward at other joints. See the racing sequences I linked to in this current thread (click here).
And observe the movements of this little stick figure animation absorbing moguls:
But EXCESSIVE breaking at the waist can result from an equipment setup problem. Specifically, boots that are too upright in the cuffs make the shins too vertical, moving the knees back, and causing the hips to move WAY back when bending the knees. In this case, the skier must exaggerate the reach forward with the upper body and/or arms, to remain in balance.
In moguls, even if you are set up correctly, if you don't move your feet forward as you hit the bump (as the stick figure animation shows), your feet will slow down and your momentum will throw your upper body forward. That "reverse bicycle pedaling" action is critical for fore-aft balance in bumps!