Rocking your foot to balance, and having it go outward isn't exactly... odd. Look at most weightlifters, even in the clean you'll notice movement in that direction its a much stronger base to have weight slightly on the outside.
If you are truly concerned, workout barefooted more. It will make your feet stronger overall. It could just be that you've worn down your shoes in such a way for that to occur. Speaking of which what type of shoes are you wearing when this is occuring?
And on another curiousity level, what is your concern? Are you having a hard time balancing?