Some Interesting posts regarding Subprime.
Tim O'Reily: Subprime Loans Glossary
I am not sure how Automated Underwriting really played in the over evaluation of the Housing Market. I believe this swift ramp up was the result of low interest rates backed by very creative loan programs especially Stated Asset Stated Income or ( liar loan as the washingtonpost's Steven Pearlsteing describes it.).
The washingtonpost has 'No Money Down' Falls Flat
I don't agree with item D Piggyback loans were not (at first) a means to no down payment but rather a way to escape Mortgage Insurance. One could argue that it was a lie to the Investor because there was more then 80% loan to value relating to the borrowers actual liability.
The New York Times offers The Subprime Loan Machine
Little bit of history relating to how AU came to Subprime and its ramifications. Although A paper ala Freddie Mac, Fannie Mae had AU years before.