I had the same thing happen to me over the last week or so. It has never happened before. I live in the south, but have about the same setup as you. The girls are outside all day, the lights go on in the hen house at about 5pm, off about 10pm, back on about 6 am. If I am not working, I check for eggs several times daily, and I have recently on two occasions found hens on the nest when I tuck them in for bed. Last night I found one of my serema hen's was on the nest and laid about 9 pm.
I kept their light on all night for a couple of nights that it was supposed to be very cold to help with heat in the hen house, so I don't know if that had anything to do with mine or not.
I have also got up early and let them out for the day and found eggs already laid. It is odd that several hens laid during the night unless they got out of sync for some reason.
