Silver tarnishes by the formation of a layer of Ag2S (tarnish). This layer of tarnish has a relatively uneven thickness and undulating surface at a microscopic level. During tarnish formation, Ag has been consumed from the nice shiny coin surface to build the tarnish layer, and the consumption of silver metal during this reaction is not perfectly even across the coin surface, forming ndulations, pits and a host of other surface imperfections.
When the tarnish is "cleaned" using the galvanic reduction technique (electrochemical), the sulfur is expelled, and the silver from the tarnish is transformed back to it's pure metal form and deposited on the coin surface. However, the silver remains on the surface of the coin in the form of microscopic rough particles, sitting on a now microscopically pitted and uneven metal surface.
This rough silver surface, covered with tiny silver particles is now much more porous, with a lot of tiny crevices, hollows and pits. So, the effective surface area has become relatively much higher, and the crevices and gaps are more conducive to collecting or absorbing any moisture, gases or contaminants that may be around (like a big sponge). Thus, re-tarnishing will occur at a faster rate.
