I agree that your chickens are probably eating eggs.
But I'm not sure it was caused by feeding them eggshells. There are many other things that could be involved, even though it did start right after you fed them eggshells.
Maybe, but maybe not. It's probably worth spending a bit of time checking for causes and trying to break them of the habit.
--Make sure they have enough calcium, so the eggshells are nice and hard. (This avoids eggs breaking by accident. A dish of oyster shell available free-choice is often a good idea, so the hens can eat as much as they need.)
--Make sure they are getting enough protein: at least 16% and possibly up to about 25% in their food. If they get many low-protein treats they may need an even higher rate of protein in their basic food. (Egg is a good source of protein. Sometimes egg-eating is caused by chickens needing more protein.)
--Try to encourage hens to lay IN the nestboxes, rather than on the floor. (Eggs on the floor are more likely to trigger egg eating. New layers often lay a few eggs in the wrong place and then figure it out for themselves, but in general it helps to have comfortable nestboxes, maybe a bit secluded or darkened, with some fake eggs in them.)
--Collect eggs as frequently as you can. (They cannot eat eggs if the eggs are not there to eat.)
--Provide plenty of fake eggs, in the nestboxes and maybe even one or two on the floor. (Every time a chicken breaks a real egg, she gets to eat the tasty stuff inside. But every time she tries to break a fake egg, she bumps her beak on something hard that does NOT give any tasty treats. Some chickens will eventually give up trying to break eggs, if they have access to fake eggs but not real ones.)
--If you notice one particular hen that regularly breaks open eggs, try penning her separately or getting rid of her. There is a chance that will stop it, if the others haven't yet learned it from her.
Some of those are things you are probably doing already. I was just trying to give a fairly complete list that's all in one place.
I would probably check those things, and then spend a week or two collecting eggs as often as you can while leaving fake eggs around. If it doesn't help in two weeks, then start thinking about whether to replace the flock.