One reason could be that she was inspired by the concept of 'going barefoot' for health reasons. Some people believe that walking barefoot can improve circulation and strengthen the feet. However, she didn't consider that it's not appropriate in a store environment. Another possibility is that she was following some new - age or hippie - like lifestyle where going barefoot is seen as being more in touch with nature. But again, this doesn't take into account the store's rules and the general public's expectations.
She might have lost her shoes. Maybe she accidentally left them somewhere in the store or they got stolen. Another reason could be that her shoes were hurting her feet so badly that she couldn't bear to keep them on any longer.