Seems a little young for going broody but sometimes when one hen is broody -the others will 'add' their eggs to the broody nest...
What do you have in the nest box? straw, hay, paper, shavings? You may want to try something differnet..
I would check for mites/lice on my hens. Go ahead, treat your nest box for pests and add fresh filler... Check the top for wasp/dirt-dobbers. Make sure you don't have a snake or rats that could be harrassing the girls.
Is your nest box near a door/feeder/waterer/window? Is it busy around it --compared to the floor nest? How about lighting? Is the nest box in the bright sunshine and the floor nest dark and quiet???