I may have figured it out.  Just read a thread about chickens roosting outside at night when it is darker inside the coop than outside, when they decide to go to bed.  There is an ice ridge outside the coop door which prevents the door from opening as far as it usually does.  Thus, its darker in there than it usually is.  Some chickens are afraid to go inside a dark coop because of the scary unknown and thus stay outside, instead.  That makes sense.  My bantam is always the last one to go inside, playing outside as long as possible, darting in and out of the coop, and by the time she decides to go in, it is probably spookily dark inside the coop, so she stays out, instead.  Makes total sense to me.  I will melt out the ice ridge with hot water tomorrow.