Like a small child that suddenly refuses to sleep in their bed, you need to look at what may have frightened the child to provoke the behavior.
So it is with chickens suddenly choosing to huddle in the run rather than go into their coop to sleep. Something is putting them off.
This happened a few years ago in my flock after a pullet died suddenly during the night. The other three refused to go into the coop the next night until I took them in and comforted them, as you would a small child, showing them there was no longer a scary monster waiting to kill them, too.
It could be mice or rats visiting at night. It could be a snake. It could be coop mites which only show themselves at night and crawl on chickens and suck their blood during the night, resulting in plenty of discomfort and even anemia. Or it could have been a predator scaring them once during the night, and now they don't want to go back to the frightening scene.
You need to investigate the above possibilities at night with a flash light and start ruling things out. If you rule out everything but a one time predator visit, then you can address that the way I handled the death of the pullet scaring my survivors. You may only just need to comfort and reassure them for a couple of nights and they'll return to normal.