I don't have any idea. I never sat in it and it may have been as American Mother stated, just very ill-fitting on the horse. It could have been the way the stirrups hung, I do know that makes a difference to me on various western saddles.
As you note, this can be because the stirrups are set wrong - i.e. if the stirrup brackets are too far back, you will tip forward. Also can be something like too much stuffing in the lining (the part of the saddle that touches the horse's back under the cantle). Or if the gullet is too broad in front. Or if you are using a lift back pad when you shouldn't. Some saddles are designed to push you forward and down onto the knee rolls.
I have the opposite problem - my mare has a dip in her back and very, very prominent and narrow withers. I have to make sure that I use a lift back pad or I look like I'm riding up hill. Also, I have to have a saddle with a very narrow throat so that the lining rests just below either side of her withers instead of further down her sides - protects her from saddle sores.