I think Fred's Hens is very much on to something. My free ranging jungle fowl go to considerable lengths to lay eggs in the nest of other hens. They are putting there eggs in more baskets spreading out risk increasing odds at least some survive. They seem only to lay in nest before incubation begins. Younger hens seem more prone to lay eggs in nest they will not incubate themslves. Domesticated birds may have simply been selected for behavior over time as it makes egg collection easier.