^^^ US Government compelling banks to meet quotas of home loans to high-risk people (Community Reinvestment Act) is what got the economy into this mess.
Issuing mortgages to large numbers of people with low and unsteady incomes (many of whom had no real chance of repaying them), combined with the Federal Reserve System pumped up the money supply so as to drive interest rates down to artificially low levels, made home ownership and housing construction looked like really great investments. Without government interference interest rates would have remained at market levels, the housing bubble would not have grown to the enormous size it did.
The government, both parties, loved that home ownership was increasing, especially among minority voting groups, however when the Fed stopped (about 2006) expanding the money supply and interest rates began rising, many borrowers found that they couldn't afford to keep the homes they had been lured into buying and many builders found that the housing projects they had started couldn't be completed because demand had died.
President Bush tried to shut the whole thing down, but he didn't have the political capital in both houses of Congress to do it, it was likely too late anyway. Banks had insured their loans through the big insurance companies, when the loans began to fail in large numbers the insurance companies were looking at collapse, this lead to the first of Bush's big bailout outs
Institutions that had invested in mortgage-backed securities discovered that their portfolios were nearly worthless. Firms collapsed and the stock market plunged.
The Government still hasn't gotten out of the mortgage market, repealing the Community Reinvestment Act (and the 1938 Federal National Mortgage Association), or abolishing the Federal Reserve.