Is it hot where you are?
I've noticed roosting is sometimes a "cool off" position depending on the temperature around the roost. If it's shady and well ventilated, maybe a bit breezy from an open door, etc. the open air underneath them seems to feel nice.
It's not my birds first choice, but if we don't soak the ground in the shade on the hottest days, and if there's a bit of breeze, they like it.
If it's not that, and it's not hiding from a cockerel or bully as someone suggested... then I'd start to be worried about her health. Roosting can be an airway clearing position, when they hang their head down. Have you seen any discharge from the mouth? Is her crop empty in the morning? Does she make a whistling sound when breathing?