The phenomenon of the humidity (water) that permeated from the coating is ionized with intervention of ion impurities and electric current, and reacting with a resistive film.
By this chemical reaction, erode the resistive film and resistance increases. When extreme, will be open.

An oxide in the ceramic rods ionizes with humidity or voltage and a cation moves to the cathode, and an anion moves to the anide, and react with the resistive substance and erode the resistive film.