Saddle slip may not be the result of an ill fitting saddle or asymmetrical back muscles. A recent study has shown that it is often a sign of hindlimb lameness. Saddle slip, in which the saddle slips consistently to one side, is a well-recognised problem in sports horses. It can occur for a variety of reasons, including asymmetry in the shape of the horse’s back, riders sitting crookedly and ill-fitting saddles.  Sue Dyson, Head of Clinical Orthopaedics at the Centre for Equine Studies, had also observed that saddle slip may occur because of hind limb lameness. See link for more info on the recent study Saddle slip study