Actually I'm no expert, but I'd say it was caused by the monetary policies of Greenspan.
Neither Greenspan or Bush had anything to do with the housing bubble... That was caused entirely by Congress passing laws designed to encourage home ownership by people with lower incomes, and simultaneously allowing those loans to be packaged and sold as investments which encouraged the banks and investing companies to give riskier and riskier loans. Both laws had the intended effect, with disastrous consequences. People who had no business buying a house were told that buying houses with loans they could not possibly pay off was somehow their right and the realization of the American Dream, and investment managers got rich trading these worthless loans. Given the inevitable outcome when adjustable rate mortgages hit the end of their low-rate period, it's clear that congress got exactly what they intended with these laws - people who had an income that could not support home ownership were kicked out of the homes they never should have had in the first place, and investment companies were stuck with worthless pieces of paper and rendered insolvent almost overnight. The laws couldn't have caused more damage if they were specifically intended to bring down the mortgage industry (reference the tobacco industry which has survived numerous legislative attempts to destroy it).
It's easy to assign blame for the mortgage crisis and resulting economic disaster, but there were a couple of laws pushed through by Congress without Presidential influence that directly encouraged and enabled the process. And those laws were passed while the Democrats were in power. this should not be a surprise, since "a home for every family" was the liberal rallying cry that led to these laws being passed without serious opposition. It's typical liberal koolaid, making terrible laws based on an unrealistic dream of how things ought to be. Well, people who can't afford homes should not be enticed to buy homes they can't afford with loans they can not repay. It's as simple as that and no amount of pretty speeches about rainbows and ponies will ever change that.
