For the answer to the question above,
when a financial firm securitizes a mortgage, it means the sellers bundles them together and sells them to the investors.
In those years, they sell a house to an individual even though he can't afford it or they can't pay for it at all. So it will become a bad debt. Then they bundled this mortgages/debt and sells them to the investors. Which cause the collapse of the economy on those years.