Don't wait so long to cut off this behavior. The sooner you cut off that behavior the easier it will be to do so. REally good trainers don't wait for the action to be really noticeable. They catch the horse when he very first starts the slightest bit of putting his thought into action.
My guess is that there is a place where she is acting out her desire to run past you to rejoin the herd, but she hasnt' gone actually past you yet. Still, she has encrouached on your space, so before she gets very far, swing the line back into her face and move her back, and back it up with the crop if necessary.