My guess would also be that you have a bird laying a weak shelled egg and it is getting broken as it is laid or when stepped on by other birds. It is then fair game to be eaten in my opinion. I have had this happen before and once you identify the bird with the shell quality problem, you can either remove from the flock or try to supplement her calcium intake for a while with a human calcium with vit D tablet crumbled on a favourite treat each day for a week and see if her shell quality improves. Calcium deficiency can be caused by feeding too many treats like scratch or corn or even green veg like spinach and kale which can inhibit the uptake of calcium and lead to weak egg shells in some birds or it can be a shell gland problem. Do you already provide oyster shells free choice? What do you feed your flock? Some whole grain feeds can lead to a calcium deficiency depending upon how you feed them.