This is not a question I have a good answer for. Tyler Cowen posts a link to a paper from an NYU Economist and a Federal Reserve banker analyzing the last 5 years of subrprime loan performance.

What's so odd is that vintage 2006 subprime loans went delinquent and are foreclosing much faster than any other earlier year without a very clear reason why. This was happening while lenders were recognizing that high loan to equity borrowers were a larger risk and were raising pricing accordingly (but probably not enough.) Variable rate loans and low document loans don't actually seem to be the issue that drove the changes either.

I'm going to be very interested to see what explanation can be found as more look into this.