Well…when powered up does it move from one side to the other with no gap? If that is the case then it is just dragging harder on one side when moving it by hand, there is regen braking with parallel wired steppers.
If is it always sitting kinda crooked, loosen up all the center assembly connecting bolts. The bolts that connect the the x and y to the z. leave it on the rails, take the bolts most of the way out and put it back together. While your at it make sure the z rails are parallel by laying it flat surface and make sure the rails touch completely.