Probably the chicken keepers worst nightmare!!…. I’ve had this problem crop up over the years from time to time and its really so difficult to stop………Its also infectious as once one hens starts next thing the others will follow … thinking they are missing out!
What causes chooks to do this beats me especially well fed and free ranging…But I tend to go with the idea that if a chook stumbles across a broken egg or soft shelled egg and it tastes good, then trouble will follow

I remember spending a lot of money on French Wheaton Marans a good few years back they were a nightmare once they started laying just ate every egg going……..I tried all the usual things that I had tried over the years and as usual it didn’t work long-term.
However in desperation to get hatching eggs and one year later of egg eating

I decided to give the birds a final chance and it did work. Certainly not expecting everyone to do the same but this worked for me!!…..I purchased a £89 6x4 shed from B&Q (Only because all the other houses in use) covered the window over so the shed was in darkness……situated the nest boxes away from the light of a cat flap sized entrance door….reduced the entry size of the next boxes with ply…………. positioned the roosting perches not too high and near the entrance so they had just enough light to see where to roost.
And finally I built a small attached fox proof covered run so the hens had access to feed and a dry area to chill out when they wanted and from here had day access to free ranging at lib……I also ordered china eggs from Carrs Billington (Farmers Merchant) and placed a couple in each nest box and hey presto 3 weeks after they were introduced we had a regular supply of eggs…….It wasn’t a cure but a preventative as some days eggs had the odd chip out of them I guess that was a hit or miss in the dark verses a real egg or a china one LOL.
Hope this helps a tad and good luck
Dave