There is another explanation: The erector tube inside all riflescopes moves under recoil because it is not fixed inside the scope, if it were fixed you wouldn't be able to adjust your zero. It is held in place against the turret feet with spring tension. It is possible that the inertia of your firing mechanism is enough to override this spring tension resulting in movement of the image in front of the reticle (or apparent reticle movement). The same thing happens with recoil, you just can't see it.
