Yup. Its almost always nutritional.
All the curtain, roll away nest boxes, mustard, etc, are just ignoring the health problems in your birds. Fix the feed, the problems go away.
Same here - I like to stick eggs in the pocket of my hoodie - and that means every time I bend over and forget there's eggs in there I break a bunch (they fall out) - they tend to eat the whole thing. (and I still don't have any problems with them eating eggs they're not supposed to eat)