Homeowners wanted better homes than they could afford. Mortgage brokers
encouraged homeowners to take mortgages that would reset to payments that the
borrowers might not be able to pay because the brokers got a commission for closing
the deal. Appraisers thought the real estate boom would continue and over-appraised
house values,getting paid at the time of the appraisal. Originating institutions (like Countrywide)quickly sold the mortgages to investment banks and other institutions. Investment banks created CDOs and got rating agencies to help design and then rate
the new CDOs, with rating agencies making big profits despite conflicts of interest.
Financial engineers used unrealistic inputs to generate high values for the CDOs. Investment banks sold the CDOs to investors and made big profits.Investors bought
the CDOs but either didn't understand or care about the risk.Some investors bought "insurance" via credit default swaps.