Definitely related to the alloy of the gold. A quick Google suggests it's rare but not unheard of, and much more common in humid climates. It's not super fragile, though it can apparently be removed similar to polishing silver (and may be silver itself causing the tarnish). The crown, pushers and caseback are unaffected, which suggests a different (probably harder) alloy.
On that watch, I absolutely love it. I think it gives it a character that is impossible to replicate, and looks beautiful. I can see why they chose to leave it unpolished...