If they've roosted there before, there might be something in there that spooked them, like a rat or a snake or some other kind of predator trying to get in. Take a good close look in there and see if you can spot any clues. Did you change anything inside the coop recently?
Chickens don't usually like to go inside completely dark places, but you say your flock has roosted inside before, so that couldn't be it. Pecking order problems are another reason why some birds start roosting outside, but again that shouldn't make all of them roost outside.
It can't be getting too warm inside the coop at this time of year.
Other than the predator thing, I'm stumped.