Some wheel wobble/shimmy is natural. It is a spontaneous steering oscillation of the front wheel that usually occurs at a predictable speed when riding with no-hands. It is simply the front wheel trying to stay straight as it is being pushed along. Once you remove your hands from the bars the front wheel has nothing to stabilize it, so your arms act as natural stabilizers.
See-
http://en.wikipedia.org/wiki/Speed_wobble (In Two Wheeled Vehicles)