Its normal behavior. I am wintering 14 laying hens, they have six nest boxes but its the bottom three that are most preferred. When a hen or two are in the boxes, other hens will come and 'scream', usually because they want into the box themselves to lay their egg and they don't appreciate another hen in, what they consider, 'their' box. If there are a couple of eggs in a box, a hen will go in there just to sit on them, so another hen will scream because she wants to lay. I just put the screaming hen into the next available box. If she climbs out and goes back to screaming, I take an egg or two, put them in another box and show them to her, then put her in. She usually calms down when she has eggs of her own to sit on.