ok so this looks like the classic zbone self-penetration, basically if a zbone intersects itself any ztissue that is touching it will lose track of the collision point positions or something and pull the fat/skin all around. there a few solutions but my favorite (and easiest) is to sim everything in one go, basically just take your fat/skin into the muscle sim. the other solutions would be tracking down the self penetration and fixing it or just adding a bunch of attachments from the muscles to the fat/skin