Is she bored? Sometimes horses that are kept in barns all the time will get bored and will start swaying back and forth (and certain, more high-strung horses/breeds are more prone to do this). They'll do it for hours. It can become such a habit, they'll do it even if they're let out. It can take some frequent interaction and stimulation to break them of it.
I assume chickens, like horses, are creatures of habit, so even if you can figure out and remove whatever caused her to start going into circles, she'll probably need something to break the habit. I mean, even if another chicken is the problem, she can be so hung up on circling that she does it even when alone (just like a horse continuing to sway in a pasture that it's free to run around in).
Have you tried, when she's alone, giving her a toy to see if she will get more interested in it than in circling? I've seen on here people recommending pine cones, bells and mirrors as things that chickens find fascinating and that they like to spend a lot of time playing with. Maybe if she finds a new way to occupy herself, she'll forget about circling and will break the habit.
