As Tiger said, it could be you have a very well-trained pony. If so, great. Fixing yourself is sometimes easier then fixing your horse. However, this usually isn't the case with backing up without a 'command'.
She is most likely doing it out of rebellion, especially if she only does it when you are asking her to work or go somewhere she doesn't want to. She doesn't want to listen and doesn't take you seriously. You may have to get tough of her. Tap and click lightly at first. If she doesn't respond, kick harder. If she doesn't respond to that, use a crop. If that doesn't get her moving, or she rears up, backs harder, or otherwise misbehaves, you may need a more experienced rider to help you.