It's one of those things that makes us scratch our heads wondering. Sometimes they're trying to sort of figure out what it is, sometimes they are just "rearranging the furniture" in the nest box. The best I've found to counter this is to gather the eggs as quickly and often as possible.
Chickens are not very smart and do not usually figure out that they lay tasty eggs... But then once in a while one breaks and becomes "smart". You should collect the eggs regularly and buy lots of those fake wooden and plastic eggs to trick those egg eaters.