I was having problems with that too, but I don't think my chickens were intentionally breaking eggs (though they were happy to eat the eggs that broke). I suspected that the eggs were accidentally cracking when the chickens laid the egg after I actually saw an egg get laid - the chicken stands up and the egg drops a few inches. My chickens like to push the pine shavings around and sometimes get down to bare plywood in the middle of the nest box, so the egg would've been dropping on the bare wood. I theorized that if the egg cracked when it dropped, then the chickens stepped on it as they left the box (I have some heavy bodies birds) the eggs might break all the way (the shell wouldn't be as structurally sound as an uncracked egg, so wouldn't withstand the weight of the chicken as well). So I started "fluffing" the pine shavings back into the center of the nest box ever time I collect eggs - and I haven't had any broken eggs since then!