The crash was a result of financial pros selling trillions of dollars in investment papers backed by mortgages. They were benign in the beginning because they used solidly backed mortgages. But the wealthy international buyers wanted more. So Banks told the mortgage companies they controlled to drop standards so they could get them. The investment banks were not at risk since they immediately packaged mortgages breaking them up into many of the investments.It was free money for them. As the House investigation showed, the rating agencies were entirely bought off by banks.
If you were around in 2008 you would have received a dozen emails a day trying to get you to remortgage or take out a mortgage to pay off credit cards and do home improvement. The rates and terms got lower and lower as time passed . They were ridiculous. Note what is missing, Freddie . Did they contribute? Probably, but not much. It was at the hands of the investment bankers who walked off penalty-free.
The bankers lowered standard until the mortgage backed investments were worthless. I suppose we could go into SWAPs which was insurance of the mortgage securities. They sold so much of that, that all the cash on the planet could not meet what they sold.