Yep, the crumbling is due to the constrained verts that keep the straps from sliding off her breasts. The higher the collision thickness (Is that the correct term? I don't have Poser open right now.), the more crumbling. I've found that in most poses, the straps will stay on the shoulders even if those pins are removed. They'll just not stay on top of the nipples.

-- I'm not mad at you, just Westphalian.