I don't know, some horses are just buggars to fit like that...or maybe I need some lessons on cinch tightening! Haha. We have one foundation bred mare who is wide-wide-wide, but she has narrower shoulders than the rest of her body, and virtually no wither (real winner in the conformation category...LOL. Not!) and I swear to you that I always check her girth three times before I mount. Yet I've had the saddle climb her neck twice already (one of those times I actually bailed since it went so far up....heh) and that things seems to loosen itself every single time, no matter how tight it starts out.
Could have to do with saddle fit I suppose...or my cinching skills, but its happened to other people when riding her too, so I vote funky-built horse! Hehe.
I go out and ride with my friends sometimes, just for giggles, and she has two wide mares like that...I have to use my Won non-slip pad, neoprene cinch, and it's a good fitting saddle but it feels like I'm riding jello...and I slip all over the place! Lol!
Once I was in a lesson on my backup horse Bailey...I went hauling ass into a barrel, went to turn it, and the saddle slipped right to the side. Luckily Bailey had the sense to stop and I swung off.