All right, from the top:
30 some years ago you had to have a down payment and a good credit rating to buy a house. You had to essentially prove you could afford it, your mortgage couldn't be higher than 30-40% of your income unless you forked over more money on the down payment.
Then the gov't changed all that, basically did away with all of it so that any swingin' dick could buy a house. They created Adjustable Rate Martgages (ARMs) that were initially very low but would accelerate the rate over time depending on the prime rate. Fannie and Freddie backed the loans and the gov't all but forced banks to loan money to people that wouldn't have been able to qualify years earlier. The regulators relaxed the enforcement, people took out mortgages on those ARMs deals, and anybody who raised objections or questions from either party or no party was summarily dismissed.
Bunches of sub prime mortgages were lumped together and sold as AAA rated securities when in fact they werreobviously not that secure. And all was well until the interest rates started to go up and people couldn't make their payments on the mortgages. And it all came down like a house of cards.
Were the Wall Street bankers and Investment firms responsible? Sure, but no more than the gov't who pushed this far beyond what was financially sound, and the regulators and credit ratings agencies and the underwriters and the people themselves who had to know they were buying something that they couldn't afford down the road. And of course Fannie and Freddie, who many think were the chief architects of the whole mess.