Mortgage lending standards were lowered, in order to raise the magic number of home ownership. But, with lower lending standards, there were-- surprise!-- more mortgage payment delinquencies, defaults and foreclosures.
This was a problem not only for banks and other lenders but also for those in the business of buying mortgages from the original lenders. These included semi-government enterprises like Fannie Mae and Freddie Mac, as well as Wall Street firms that bought mortgages, bundled them together and issued securities based on the anticipated income from those mortgages.
In other words, all these economic transactions were "interconnected," as the Russian economists would say. And when the people who owed money on their mortgages stopped paying, the whole house of cards began to fall.
Politicians may not know much-- or care much-- about economics, but they know politics and they care a lot about keeping their jobs. So a great distracting hue and cry has gone up that all this was due to the market not being regulated enough by the government. In reality, it was precisely the government regulators who forced the banks to lower their lending standards.
The other big lie is that this was a failure of economists and others to foresee that the housing boom would turn to bust and set off financial repercussions across the economy.
In reality, everybody and his brother saw it coming and said so-- including yours truly in the Wall Street Journal of May 26, 2005. As far away as London, The Economist magazine warned about the danger. So did many American publications and individuals. The problem was that politicians refused to listen. They were fixated on the magic number of home ownership and oblivious to the economic interconnections that Russian economists saw long ago and from far away.