In today's (Sunday August 26th) New York Times, it said that "Many government officials and housing-industry executives had said that a nationwide decline would never happen."
So, what happened?
Cheap credit which brought higher risk borrowers into the mortgage market.
Where did the cheap credit come from? Who wanted to take on higher risks at lower prices? It doesn't make sense.
Actually, very smart people came up with this seemingly unwise investment proposition. In fact, they were basically the same very smart people who lead the investment boom of dot.com companies and telecoms and the irrationally exuberant markets of the mid to late 1990s.
But too much money flowed in for too much risk and when the excesses were discovered, the money flow dried up.
A second force building the credit bubble supported by home equity values also came from smart people - they arranged a sharing of the risk so that the high risk could be put off on to the shoulders of those who had the financial assets to absorb it should things go wrong. Or, at least that was the theory.
The recurring weakness of free market financial capitalism (the tulip bulb mania in 17th century
When the fever is on the market, new sources of liquidity come into play, driving up prices as more and more players seek to profit from the mania. Then, as always, liquidity runs out; the smart money people realize that there is no "there" there and pull back. Prices fall until a stable point is reached were market values more correctly approximately intrinsic values.
Now private equity, hedge funds, and buyouts have piled on where home mortgage lending lead the way. Too much leverage, too much debt in relation to the underlying fundamentals.
So now we are in a necessary correction for a time.
The real correction, necessary, however, is to figure out some self-correcting prudence that could be built into financial markets and temper episodes of "irrational exuberance."