A double transformation effect is where skin points are subjected to the action of a joint more than once, resulting in extreme, undesirable shape changes. Double transformation effects can occur if the skin object points are also being affected by a deformer that is in turn affected by the joint’s actions.
For example, revolve a curve and bind the curve as well as the revolved surface. You will get double transformations since the surface is deformed by the skeleton, plus the revolve is computing a new revolve based on the curve’s deformation.
To counter this, delete or detach the construction history on the shape before binding.