Way too many mortgages given to borrowers who had practically no collateral to support them. Some were those wishing to own their first home, some were those wanting more home than they could truly afford (who covered this shortfall with absurd HELOC's right at the beginning of executing the mortgage), and some were simply buying expensive homes that they solely intended to flip, which they again did not have the collateral to support.
Not only were these risky loans handed out en masse, but they were given out with interest rates that weren't high enough to represent a proper Risk Premium. We all apparently thought that home values would just keep soaring upwards forever.