Laws passed by Congress such as the Community Reinvestment Act required banks to make loans to previously underserved segments of their communities, thus forcing banks to lend to people who normally would be rejected as bad credit risks.
These governmental measures, combined with the Federal Reserve's loose monetary policy, led to an unsustainable housing boom. The key measure by which the Fed caused this boom was through the manipulation of interest rates, and the open market operations that accompany this lowering.