Who caused the mortgage collapse?

From Samizdata:

What caused the economic crises?

"Greedy bankers" says some people.

There is certainly a lot of greed about. For example, the people who trampled a part time Security Guard to death at a Walmart on Long Island (as he shielded a fallen pregnant women from them) were certainly greedy. Even after it was announced they had killed a man they still did not want to give up shopping for bargains in the sale and were very angry at being removed from the store. But I doubt there were any bankers in the Walmart sale crowd - although I am open to being proved wrong.