Here is a good description about how Wall Street greed brought down the financial markets. Actually the greed went further than Wall Street, since people who selling houses to other people who had no chance to pay for them. We all knew that, and that alone should not have caused the catastrophe that has occurred. The problem seems to have been that the shrewd operators basically created mortgages out of thin air and sold them. Basically these were naked short sales. Why did the financial institutions go along with this. It appears that they are either crooked or stupid. Most of them have ridden into the sunset with their ill-gotten fortunes.

http://www.portfolio.com/news-markets/national-news/portfolio/2008/11/11/The-End-of-Wall-Streets-Boom?print=true