i think its a natural behavior of chickens .. it assures a fast clutch of fresh eggs and at some unspecified number of eggs will make a hen go broody ... you can dispurse 'marked' eggs to other boxes to promote their use if its a problem, change out the marked egg every 2 weeks or so i usually date them as the mark ... but, with 5-6 layers or less, if you keep the nest box theyre using in great shape, plenty of soft grass etc, AND using a box with plenty of room, it generally will produce the cleanest eggs on a daily basis rather than a bunch of boxes with 1 egg in them .. been my experience ..of course if you got 12+ layers for example, your goingto want to promote the use of other boxes ...