Sometimes it is nutritional, environmental, or just because they can. I've found that chickens prefer more protein than is commonly found in levels of layer ration (16%). They usually find it on range or you could start by mixing 1/4 game bird crumbles or starter/grower with you lay pellets/crumbles. Make sure you mix enough oyster shell in the feed. I mix instead of leave a big container of oyster shell ever since I had a hen get crop bound from eating too much of it.
Hens also like a nest area where they won't be harassed by other birds trying to edge them off their nest. They like it dark with as little noise as possible. I hung curtains over my nests made from shade cloth, put a curtain over an East window, put some realistic ceramic eggs in the nests, and it helped very much.
Sometimes there will be a troublemaker in the flock the other chickens see breaking and eating eggs and it becomes competitive, learned behavior. It also helps to get out there in the morning and check for eggs after they've been out for a few hours. Hope that helps.