The physical causes which lead to the formation of milk spots are correctly described above. The underlying reason (which allows the physical causes to continue to exist in the coin-making process and to continue to cause the formation of the milk spots) however is the abyssmal if not non-existent quality control systems and procedures employed by the RCM. I saw a press release the RCM released some time ago in response to inquiries and criticism on the subject of milk spots on their silver coins. The essence of the press release was "Yeah--we know. So---what's your point?" It appears that they just couldn't care less. The milk spot problem seems (at least within my personal experience, and I've stacked a lot of tubes of silver Maple Leafs over the years) to be as prevalent today as it was many years ago.
The milk spots are the main reason I moved out of the Maples and into ASEs and I've never regretted that decision. Probably should have done so years earlier. Oh, well... My decision-making abilities may be clouded and run slow, but at least my hindsight is 20/20.