Reflex mirrors are front-silvered (first surface mirrors) in order to avoid secondary or "ghost" reflections. A thin layer of aluminum (or aluminium, depending on where it's done) is evaporated and vacuum-deposited on the surface of the (usually) glass substrate. "Thin" doesn't really begin to describe it; the aluminum film is only a few atoms thick, and part of that film oxydizes almost immediately to a clear sapphire coating.

That thin film is relatively easy to damage, as these things go. And that wouldn't necessarily be much of a problem — a few microscopic missing chips here and there won't affect the overall image much until there are a lot of them. The real problem is that aluminum sticks to itself a whole big bunch better than it sticks to smooth glass. That means that there is a severe danger of high-velocity, high-pressure air getting under the film through a small, naked-eye-invisible flaw in the film and lifting a larger and much more significant chunk of the aluminum off of the glass. It would be like peeling a Clementine or Mandarin orange — once you reach the threshold of force required to start the process, it doesn't take much to keep it going.

(On reflection — no pun intended — I suppose I should have described the process more correctly as "high-pressure, trapped air under the film, with high-velocity, lower-pressure exapanding air travelling over the film". This site is, after all, home to several people who want a technically precise explanation rather than the easier-to-visualize but less-precise one. Choose whichever picture helps you most.)