If you want to know what's going on watch your chickens. Get out there a few minutes before they normally roost for the night. Observe the action. Chances are the BO is throwing her weight around. I have a BO that thinks it's great sport to haze the older, weaker, younger, more timid chickens as they try to come into the coop to roost. I have a Cream Legbar hen that likes to bully a few easily intimidated hens during the day in the run.
Far from stationing myself as run and coop cop, I do discipline these two bullies whenever I see them pull their macho little stunts. I keep a fly swatter handy and all I have to do is raise it up and show the bully, and she suddenly decides to stop playing her game. They each got a prior conditioning swat, and they will never forget what that swatter can do to fuzzy little butt.
Bullying is the most common reason for chickens suddenly preferring to sleep somewhere besides the coop. But rodents, snakes, and coop mites can make it very unpleasant in the coop, so chickens will not go inside. You may want to rule those things out.
