This happens in both flat backed and high withered horses so see High Withers - but the horse will invariably have a well defined wither
This usually happens when the horse carries a lot of flesh on its shoulder and the wither is proportionately narrower. It is not common in dressage saddles – the further forward cut the saddle is the more this is likely to happen and it happens to a greater or lesser extent in many horses where the withers are fairly prominent and the shoulders quite broad and well muscled. Many Eventers and Show Jumpers suffer from this - it is not always seen as the saddle slipping back significantly but the saddle is moved slightly across the back every stride, as the shoulder comes back and this creates soreness up against the spine.
Changing the position of the girth straps and placing them further forward is often effective.
Saddles with this feature are
A breast girth or hunting martingale is recommended as a safety precaution
A significant advance in the stability of the saddle is a feature seen in many of the NSC jump and General Purpose saddles. We have created a design where the point of the tree is left on top of the panel “floating” - so that the part of the flap and panel where the riders leg sits in a shorter stirrup scenario can move quite easily without moving the rest of the saddle
The Advanced Competitor, The Event /Hunt saddle Mark2 and Mark3, The New Grandee, The Nyala, The Pony Pro
See also Herring Gutted Flat sided No Girth Groove and FAQ Is My Horse an Extra Wide