Cygnets are still intact and thriving, Popeye and Penny keeping them close and ever watchful of potential danger. Penny is entering the pre moulting stage and pulling at her feathers, I think there is some irritation and possible parasites that need expelled with the old feathers.
Sitting on a nest for nearly 7 weeks is bound to allow some pests to get onto her body.
She was doing a lot of picking and had a vigorous bath. ( short video below ).