Skin shedding is generally a sign of overeating. I believe
Vitamin A in large amounts can cause this problem.

If you are feeding a variety and the water is clean and
the turtle is otherwise healthy I can say with 95% certainty
that you are overfeeding and thus causing growth which is
too rapid. The scutes on top of the shell will peel off in
sections. It can look really troublesome as they are loosening
because everything gets under them in spots. This is how they
grow.

Hope this helps,
Linda