I'm no economist and haven't heard this anywhere outside of my own head but do have a theory:
3-6 years ago the economy was pretty much in the dump and there were ZERO new vehicles selling (or being manufactured). It stands to reason there is now a shortage of late model used vehicles. The economy has picked up in many areas and now there is heavy demand but little supply - So, up go the prices.
I'm very intrigued by the current used car market. I find it funny that all of the sudden there are gleaming franchises selling used vehicles (Car-Max for example). I for one think this business model is going to fall flat on its face once things "normalize" in the used car market. The supply/demand used car thing has always resulted in dimly lit lots in a crappy part of town moving vehicles. I just don't see the long term profitability in huge, clean showrooms with perky young ladies selling used cars.
We'll see....