Dilute a quart of laundry bleach in a gallon of water mixed with a cup of TSP substitute. (Used full strength, bleach can chemically burn the wood.) Brush onto the deck surface, and rinse before the solution has a chance to dry.
This occurs only on finishes that form a film, such as solid-color stains; penetrating stains don't peel. There are many causes of peeling—poor prep work, wood decay from rot or sun, water vapor getting underneath the film and lifting it, UV damage to the finish itself—but the only cure is to remove the peeling stain and start over on a sound, clean surface.
It's the inevitable effect of the sun and foot traffic, but it's also a good indicator that it's time for a new coat of penetrating stain or waterproofer. To find out, test the wood by pouring some water on a shady spot of your deck and seeing how long it takes to soak in. If the water disappears in less than 5 minutes, the wood is ready for a new coat. If a finish seems to be fading too quickly, use a stain with more pigment.