There wasn't any noise that she was fired or there was a problem. It was reported at the time she wanted to move back to London to be close to her children. I believe they were college age at the time. You can't be an Apple Sr. VP and live in London (especially back then).
Apple didn't try to prevent her from getting other jobs by making her "Special Assistant to the CEO" or some other nonsense, Ahrendts is on the board of Airbnb, but hasn't did anything notable in the private sector since. She still has Apple retail pictures on her social media.
She was super rich before Apple. Maybe she got into the job at Apple and realized she'd rather spend her life working with non-profits (Save The Children, Charity Water, etc.) and being close to her family?