in the case of toppling in rotational dynamics why the normal reaction shifts from its original position

The normal force shifts from its original position so as to balance the torque of friction....U can take an example of a cube moving down a rough incline with constant velocity.Since it is in translation and rotational equilibrium as well, so the normal force exerted by the incline shifts (obviously has to shift) downwards so as to balance the torque of frictional force.Hope I helped u....:)