The official report on the cause of the housing bubble from the Appraisal Foundation:
http://http://www.appraisalfoundation.org/s_appraisal/bin.asp?CID=321&DID=1270&DOC=FILE.PDF
What were the root causes for this huge increase in housing prices? There are three significant reasons. First, the cost of money was at a historically low level. Interest rates, which had been as high as 16-18% in the late 1970s and early 1980s, were at 5% or lower. This inexpensive money had two impacts: (1) it allowed current home owners to buy a more expensive house than they could previously afford, which encouraged them to trade up and (2) it allowed people who previously could not afford a home to enter the housing market for the first time.
Secondly, because of the significant annual increases in U.S. housing prices, lenders became more complacent with their underwriting and lending standards. The rationale for the lowering of lending standards was that even if the loan did not perform in the future, double digit annual price appreciation ensured that the collateral would be worth more in subsequent months and years then it was at the time the loan was made. This was viewed almost as loan insurance by many lenders. Accordingly, many people were approved for loans that would have either been approved for a much lower amount in the past or even been denied a loan.
Third, for the first time the private sector, primarily Wall Street, became a major factor in the secondary mortgage market through securitization. This replenished mortgage lenders with additional funds to make more loans.