Their mothering instincts might be kicking in. If you allow enough eggs to accumulate, you may even find one becomes broody...
Mine laid them all randomly for the first year, and then I didn't collect for a week over vacation, and all of the sudden they started laying them in certain areas, and basically building clutches. Seeing a pile of eggs seems to engage something in their little bird brain, which seeing a single egg or two doesn't. Once you get to 8 or 10 eggs in a pile that seems to trigger them. I introduced a nest box, and once I got it right (lining it with thin dead grasses seems to be preferred), they transitioned to the nest box on their own, broody instincts kicked in, and one of my hens raised some chicks for me! (The first time around she failed, the second time around she got it right).