Chickens are scavengers and eggs are high in nutrients, so eating them is not surprising. It probably started very innocently when an egg got broken by accident.... maybe a slightly thin shelled egg that got stood on. Once they realise that they taste good and it's worth breaking the shells on purpose, it's quite difficult to stop them. The most successful solution is to install roll away nest boxes, so that the eggs are automatically removed to a separate compartment as they are laid. I have read of people filling egg shells with hot mustard but there seem to be mixed thoughts and results on that. Collecting the eggs regularly and replacing with dummy eggs may help, but you have to be around the coop all day so that you can catch them.
Some people believe that it is a lack of protein in the diet, but I think if a hen finds something she really likes, she will eat that regardless of whatever else is offered or how balanced her diet is. Boredom can play a role in it sometimes....for instance, when they are confined to the coop in bad weather or don't have enough space in the run or ground to scratch. If they are confined to c run, try hanging up a cabbage that they have to jump up an inch or two to reach. Give them a sod of grass each day or a few shovel fulls of muck from a compost heap or a bag of raked up leaves or anything that they can make a mess with. I'm currently weeding my veg plot ready for spring sowing and I am throwing the weeds into the run for my hens to scratch through and eat. I spend 10 mns each day at the task which means they get a little fresh greens each day and of course any worms or bugs that are in the soil attached.
Unfortunatel, once they have acquired the habit of egg eating, it is very difficult to break them of it and other hens see them and learn it too. If roll away next boxes are not an option (They are no doubt expensive to buy but I believe you can make them yourself) then it might be worth isolating the offenders from the rest of the flock and perhaps trying the other options until they have reformed or you decide to cull.
Good luck
Barbara