I?ll take a stab at it, with the help of a book called ?What?s Wrong With My Snake?. It doesn?t mean that any of these things are the problem, but it may give you something to consider.
The reasons given are: parasites, malnutrition, infection, metabolic irregularities, tumors or usually poor environment ie humidity too low, substrate too dry, or no rough surface to rub against.
Well, I hope this helps and it?s none of the more serious things.
Cheers Artie