Actually, the root cause is the ability for mortgage brokers and other institutions in the CDO circle to sell loans immediately, no risk, and get paid for it. In an ideal market situation, those who make risky propositions for greater expected gain also have to pony up if things go south. These crazy sliced up CDOs allowed a game of "hot potato" with these loans, where everyone in the circle was able to escape the risk and get gain, except the last guy (usually a bank) who loaned money based on the supposed value of these hot items and got them as a toasted collateral when everyone else in the circle couldn't pay up.