If I understand rightly, are you saying that the zoom ring on the lens turns just by you moving the camera, for example from landscape (flat) to portrait (vertical) orientation? If so, then I'd say no, it definitely shouldn't do that.
If however, you mean that the lens zooms when you point the camera down, some lenses do slowly zoom when this happens - It is called lens creep and can affect the lenses with the heavier elements or the longer zooms. How easily then lens "creeps" is a matter of personal tolerance... If it is under warranty you may be able to have it looked at if you say it is excessive.