OK, I see it.
The ultimate problem is that ghastly "balance" feature built into the AM2001 rig. Worst idea ever. It's a fake sort of averaging of the hips to the feet positions and not at all a good approximation of real "balance". Send your complaints to Rafael Anzovin who invented that.
If I delete the second key on "Balance Right Foot" the drifting of the hips will stop. You may have moved it inadvertently.
However, I recommend you go back to frame 0, set the User Properties "Balance" and "Balance Rigid" to 0% and then delete all the "Balance" channels in the action. Then repose anything as necessary.
Posing the character's hips and feet will be far more predictable with those two "Balance" sliders set to 0. Do that at the outset for any animation you embark on with the AM2001 rig.