According to this page, "the Yahoo! Mail team implemented a filtering system intended to radically reduce the amount of spam you receive in your inbox by directing most bulk messages to your bulk mail folder. This filtering system (called SpamGuard) is automatically activated and a bulk mail folder is generated when you receive a message that our automatic filter determines was sent in bulk."
Holy crap! How can they tell it's bulk email? Yahoo, via the help of this so-called "automatic filter" is reading your messages!