First check leathers and be sure that the rider is not one sided – because that will wear the saddle more on one side, and thereafter, without, expert flocking to get it level again, it will sit to one side. It may be an inappropriate saddle – either too narrow or too wide and, very occasionally these days, it could be the tree- but it may also be a bio/mechanical issue. It is true that if a rider is one sided that can result in a crooked saddle and a crooked horse. Riders need to address their own posture if they find that all saddles start to go to one side.
In view of the enormous number of correctional pads on the market, at the NSC we prefer to advise the use of something appropriate adding extra shims wherever needed and keep the saddle as straight as we can.In the last few years we have had many advocates for the” Wide saddle”, - wider than the horse warrants, and the use of various front riser pads, saddle cloths , shims etc. to get acceptable balance. Looking at this from the viewpoint of building muscle this would appear to be the right way forward and there are cases where with educated riders, very conscious of their ability to sit straight and keep their horse straight that this approach can work extremely well. However, in practice, it is not always successful - horses like riders have a “jelly side” and we are nearly always stronger on one side than the other. It is very easy in such circumstances to find a saddle which, in every other respect is excellent, lurching off to one side.
The NSC does not like to recommend this approach except as a last resort and after having tried several different trees – we could never adopt it on the wider flat backed horse without defined withers as the rider’s safety may be put in jeopardy.