Touching of dis-similar metals in the presence of acids, in a moist environment creates an electrolysis. A better explanation might be possible if you were to tell us what it is that is pitting.
Sweat from your skin deposits salt and his can lead to corrosion i.e. pitting. Also if you get small microbial aggregations that are eating skin/dirt deposits they can produce acidic metabolic products that corrode metal.
Worn immediately next to the wrist, I would say the area would be subject to sodium (in perspiration), as well as acids. Warmth, moisture, acids, sodium, pitting.