Suddenly Sleeping in Nest Box

I have six hens that are about 10 months old and they have been perfect in every way...that is up until now.  They have three nest boxes in a row and on the other end of their coop are three roost bars each about four feet long and each at a different level.  Up until now all of the girls went into the coop at dark and all of them got on the roosts and stayed there for the entire night.  This was great because it made cleaning up the night poop very easy at it was all in one place.  Now one of the girls has decided that she wants to sleep in one of the nest boxes, which I do not like because I don't like the poop in the boxes.  For the last several nights I have gone out after dark and pushed the offending pullet out of the box and waited until she hopped up onto the roost.  That has been successful because once she has gotten on the roost she has stayed there, but she continues to start the night in the nest box.  Why has she suddenly started this and is there anything else I can do to fix it.?

Thanks for the responses.  1) All the nests are already at the lowest level of the coop and the roosts are at three levels the highest going to a point where the chickens heads will almost touch the roof.  2) I do not notice any indication that she is being picked on and, in fact, when I do go to check after dark I do push her out of the nest and she goes on the roost and stays there for the rest of the night.  3) As for broody, I guess that is possible but she only goes in the nest box at night when it is getting dark.  She is not on the nest at all during the day other then to just go in and lay her egg and then she leaves immediately thereafter.