Why is skin in my mouth coming off?

See a dermatologist. There are many conditions that can cause the skin to come off in the mouth. If this is a constant or recurring problem, see a dermatologist for a correct diagnosis and treatment.
Allergy. This could be yeast infection or allergy to oral products such as food coloring or preservatives. Stop chewing gum, lollipops, sucking candy. Change toothpaste to sensodyne. Use combination of Hydrocortisone and burt's bees beeswax product. If it persists see doctor.