Why Does The Moon's South Node Point To The Past?

My online school, The Forrest Center for Evolutionary Astrology, is blessed to have Mister Sagittarius himself, Ryan Evans, as one of our tutors. True to his Sun Sign, Ryan is an ace at asking the really big questions. A few months ago, I got the following email from him. In it, he raised an issue that could not possibly lie any closer to the heart of evolutionary astrology:

I have a question regarding the astronomical understanding of the south node. Do we consider it to be “the past” primarily due to the notion that south equals “down” equals “past” or is there more of an astronomical correlation than just that? I’ve got a scientist here asking astronomy questions.

I should start by saying as clearly as possible that I have no definitive answer for Ryan. Why should the place where the Moon’s orbit crosses the ecliptic heading south pull us toward old, outworn attitudes and behaviors? All I know for sure is that the observation is robust. It works reliably. 

But why?