a lot of things.

It started with outsource american jobs. that started the recession then Greenspan lowered the interest rate. Then come with the CDO. the financial engineering with the lower interest rate created the housing boom. With the help of CDO, the mortgage loans not regulated by the governement. Started with subprime loans, then alt-a loan, to the prime loans. What is worse is the negative arm loans. The crisis is far from over. CDS is a big problem, As a matter a huge problem but it is not where it gets started.
It is a shame that this article did not mention housing bubble. Otherwise it is a good article.