Securities are supposed to be somewhat secure. When a significant slice of the securities market turns out to be shaky, the vibrations wobble the entire system.
It appears that the US economy "printed" a trillion USD with rapidly inflating home prices and creative mortgages. It turned out to be this century's version of a Ponzi scheme of escalating value that was supposed to be underwritten by future escalating value until everyone realizes that it cannot work.
Here is a simple way to look at it....
Houses doubled in value in CA, AZ, NV, WA, and OR between 1995 and
2005. Did the wage earners wages double? No. How do you expect them to pay for the houses? Answer: they really cannot after the first few years of their exotic mortgages fade away and they have to pay for the inflated cost of the house and for the money they borrowed. The entire industry, homebuilders, bankers, etc essentially printed a trillion dollars based on the belief that people will always find a way to pay their mortgage. They convinced everyone that since the housing stock in Detroit and Buffalo was actually losing value the market must be doing its job. But upon a little inspection you will see that the "loss" in Buffalo was perhaps one-fiftieth of the "gain" in Phoenix or Las Vegas or Seattle.When a trillion dollars of US securities simply goes away it does shake the worldwide faith in the financial system.
The idea that houses will always go up and up and up in value is sheer nonsense. The people who live in those houses have to be paid enough to make the mortgage payment. Right?
Two people gain their CEO positions. One supresses wages and the other inflates the cost of a house. Does this seemable workable in the long term? The other CEO of the mortgage provider sharpens his or her pencil and creates creative mortgages. Eventually they all three butt heads and the unfortunate result is the realization that a person or persons earning 80K can afford a 200K house because that is 2.5 times their annual earnings. They cannot afford a 400K house in a hot area because of smoke and mirrors.