Birds have a fight or flight response. Generally, they fly off first if they feel like they are in danger and think about it later. Safety is getting as far away from danger as possible.
Now, when a bird feels like they are trapped and can't get away from danger, they will drop feathers. If a predator has captured them by tail or by wing, dropping feathers may help them to escape.
Our birds are most often stuck in their cages with no where to go. They'll feel trapped. The one thing that's supposed to keep them safe all of a sudden becomes a prison. Instincts kick in and their feathers go!
Thankfully, the feathers will start to grow back in very quickly! But an ordeal like that, plus growing in those feathers can be stressful on them!
Hope your RB girl is feeling better!