I don't know why they do that. Usually they are creatures of habit and will keep laying where they lay. Maybe one needed a nest and they were all full so she found an alternate place to lay. Once a hen sees an egg, she thinks it is a good place to lay,so the others joined in.
Two possible solutions for you. One, lock them all in the coop until they lay. Don't give them the option to lay under the coop. Once you break them of the habit, maybe they will lay where they are supposed to.
Watch very closely and when you see one settle in to lay, move her to a nest box and lock her in until she lays the egg. This has been successful for me but you do have to catch them laying.
In either case, put a fake egg in the nests where you want them to lay. A golf ball works well. Keep the eggs cleaned out of where you do not want them to lay. if possible put something in the way of their nesting spot so they cannot lay there.
Good luck. This one is not always easy.
but this morning I have found 15 eggs all piled up under their coop is there a reason for this is it because its cooler under there ? and is there anyway I can get them back to laying in their nest boxes which they always have done ? even my 8 year old bantam that has always gone in the nest box is under there
