If you actually spend some time researching ( i.e. reading ) about the cause of subprime , then you might learn something . There is plenty of information available .
Companies on Wall Street were making massive money out of selling the debt as mortgage backed securities . The more they sold , the more they made . Once they exhausted the supply of quality mortgages , they went looking for a replacement , hence sub prime. They were all about packaging the debt and on selling it . Making massive profits with no responsibility . Kicking the can , so to speak .
How did they manage this ? Dodgy valuations , dodgy employment and income figures of loan applicants. Absolutely criminal behaviour on a massive scale .
