It's happened from the time this (and other) boards came into existence. Some people, for various reasons, prefer to announce their departure, usually airing some grievance in the process. Personally, I pay no mind to it.
One of the funniest replies I've seen to such an announcement was "see you tomorrow". I can't remember which thread (it was years ago), but that reply struck me as quite funny. And it's true - it often seems they come back anyway.