It's a hen thing. They love crowding around one nest box, even straddling or laying atop another hen in the nest. As long as they don't have any bloody fights it's not a real concern.
If eggs are getting broken or laid on the floor, you have a problem. You might put a couple fake eggs in one or two of the unused boxes. It very often will divert their attention to those other nest boxes, relieving pressure on the favorite.
I'm getting ready for new chicks come next month and I need the coop, that includes a grow-out pen, where there are three nests to be out of commission in prep for chick occupation. So closing down this coop put pressure on the other coop that has only two nest boxes. So I created a long nest under a low perch for use during the day and covered with a poop board at night. The hens wouldn't have anything to do with it until I salted it with a couple of plastic eggs. That did the trick and now one or two hens will lay eggs in it.
Try it. If they think other hens are laying eggs there, they naturally want to lay eggs there too.