I have seen that before (a long time ago) but have forgotten what caused it. Not a 308" projectile by mistake?
Update: My trusty rusty Speer Reloading Manual #10 says that shoulder collapse is not common and can result from too much freebore when using slow burning powders and it also says "particularly in magnum chambers". Your powder doesn't seem to be all that slow for that cartridge and projectile though.
