I do not have a reason why your daughter is doing that or if there is a problem with her.  I just wanted to let you know that She isn't the only one.  My little cousin used to walk and run like that all the time.  He said it never hurt and he did it from when he was very little.  I am not sure if he can still do it, he is now a teen.  Have you tried letting her walk around with some shoes on?  It might help her train herself to walk with straight feel.  Not sure if this will help or not