5.   And it was Bush who started it.

people risk money, buying stuff they can't afford to own, and sell it for more money.
they plow the profits back into buying stuff they can't afford to own, and sell it before the ownership costs come due, and make money.
It hits them that if they started with more money, they could buy more stuff and sell more stuff.
They borrow money to buy stuff they can't afford to own, and sell it before the ownership costs come due, and pay off the financing and still make a profit.
Now the bank wants to see more of this and it encourages such deals.
Meanwhile more people are buying stuff they can't afford to own, and financing the purchase, and paying off the purchase and the financing with a higher resale.  The cost of stuff starts shooting up.
By now the banks are concerned that they are putting a lot of money and just trusting some guy to sell in a smart way, so they start buying insurance on the loan from other banks.
If that goes on long enough some eggheads declare it is the new Yellow Brick Road to Paradise and it's impossible to fail, because so many people need to succeed.

Then some day the price of stuff doesn't rise fast enough.  It doesn't SINK, mind you, it just doesn't rise fast enough.
A certain percentage of wheeler-dealer who bought 50 stuff to sell at a higher price can't sell at that higher price before the ownership costs come due.  He never planned to pay them anyhow.  He's hit with ownership costs--certainly taxes, which have priority and can put liens on his stuff-- and the financing.  So he goes belly up.  A certain percentage of operators fail.  Now their stuff is in bankruptcy, and the government agent selling his stuff is just lookign to sell it.

This doesn't help the price of stuff keep rising.  Operators know they can buy stuff cheaper.   This means a greater percentage of operators goes bankrupt.  This lowers the price of stuff.  This means banks start taking losses on financing.  This means their insurers are up shit creek; they haven't been paying out on bank insurance so they sold policies like flapjacks.  they dont' actually have the assets to cover them.

This would be a perfect time for operators, bankers and insurers to go out of business, since they are farkin idiots.

BUT NO, now USA will step in and bail them all out!  Because we want the Yellow Brick Road to paradise.

Stuff happens.  Housing, undeveloped land (remember Whitewater?)  stocks bonds whatever.  There's always something to inflate into a bubble.  Govt should not guarantee the markets because govt has less money than the private sector unless they print it like monopoly money.

And we are now on the hook as often as it happens.  Which will be about once a decade.