If you google "housing bubble blog", then I'm sure you'll find a much better explanation of the whos, hows, whys and whens.
But put simply, they are, in no particular order,
1) Yen carry trade - 0 interest in Japan is cheap credit to spend money in states
2) CDO - mortage banks / investbanks bundle anything they can and sell it as a "stock" e.g. student loans, property, whatever
3) The fed removes almost all necessary collateral restrictions on banks
4) The Fed lowers the prime interest rate to historic lows
5) Good old fashioned greed by all parties + cheerleading by newspapers + real estate agents + many other psychological factors
6) Mortage markets do not behave like stock markets
I'm sure I could come up with a handful more. But a bit of researching and you'll find plenty of nuggets. I such the homepage here as a great start.