Vitreous is the Latin term for that which is glass or glassy. The prefix de is rooted in the Latin preposition meaning "down from" -- as in to move down from a particular state. For example, defrost means to move away from a frosted state (not to be confused with, say, Massachusetts which is more of a snowy state than a frosted state these days).
Combined, de and vitreous become devitrification -- the word describing the process in which glass becomes unlike glass. Specifically, when glass, which is normally non-crystalline in structure, begins to crystallize.
There are a number of reasons glass might devitrify. Most glass has a temperature range at which it is particularly susceptible. Glass manufactured for use by artists is often formulated to resist devitrification. Glass that isn't clean tends to be more susceptible to devitrification because contaminants on the glass provide the necessary place for the crystallization to start (nuclei).
Devitrification, when it forms, usually appears as a white "scum" -- or non-glassy -- surface.