There are as many answers to this as their are schools of economic thought (i.e. many), because what you are asking is in essence: what causes a recession?
I will give you an oversimplified version of the answer from two major schools of thought (which contradict each other):
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“Animal spirits.” This viewpoint, often attributed to Keynes, suggests the recession occurs pretty much just because people freak out. They get afraid to spend money, for any number of reasons, whatever creates a mob mood. When they don’t spend money, products don’t move. When products don’t move, producers have to cut back on expenses, like wages. So wages fall, unemployment rises, and the people who were freaking out say See? I was right! and then people who were calm start to get worried – the effect snowballs and things get worse. The process only stops when people stop panicking, either because they tire themselves out, because the worst never materializes, or because government intervention reassures people.
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“Delusions of wealth.” This viewpoint, often called the “Austrian school,” suggests the recession is actually the healthy response of the economy to the misallocation of capital that occured during a previous period of ebullience – the preceding boom. During a boom, when prices of assets (e.g. homes or stock) and investments (stock, business loans, mortgage loans, IPOs) are soaring, people get carelessly enthusiastic. They stop thinking about the fundamental value of the asset or probably return on the investment, and just take note of the soaring price. They assume they’ll make money not by some organic process of appreciation, e.g. the company producing something valuable, the highway being completed and improving the desirability of this property, et cetera, but just by finding someone later to sell the asset/investment to at a higher price. Usually called “the greater fool” model of investing: Sure, I may be a fool, but as long as I can sell this to a greater fool at a higher price, I win.
But sooner or later this party has to end, because you run out of greater fools, and the underlying assets or investments turn out not to be worth what’s invested in them. The capital that is invested in them is not performing – not creating any real value, and consequently not returning any real value. Worse, capital tied up in nonperforming bubble assets is not available to genuine investments that return real value. For example, every $1 invested in Twitter stock, assuming it will someday turn a profit, is unavailable to, say, some scruffy inventor who has an idea for a better solar cell. So…you get into a situation where real economic growth slackens or stalls, although the GDP may still soar on the basis of the increasing nominal value of these bogus investments, which hides the problem for a while, maybe quite a while. Nevertheless, usually at some point people wise up, and start pulling capital out of the nonperforming assets. When that happens, you quite naturally get a crash. The price of these bubbly assets implodes (i.e. real estate prices take a tumble, or the stock and bond prices fall, et cetera). This is often magnified by the fact that many people borrowed money to buy the assets in the first place – e.g. took out 3% down FHA loans to buy real estate, or borrowed on margin to buy stock. When the asset prices drops, these people get wiped out, which pushes the price further. You get a rapid disinvestment of capital, which starves businesses, particularly those that were bogus in the first place (Solyndra), and unemployment jumps up. The Austrians describes this as a necessary “creative destruction” in which all kinds of businesses that had no real justification for existing are destroyed, jobs which created less value than their salaries disappear, and profits and value that was purely paper and unjustified by the underlying economic reality vanish, turning loads of people poor – or perhaps making them suddenly realize they’re poor when they fantasized they were rich.
From this point of view, what’s “lost” in the recession is the delusion that things are going swimmingly according to some old stupid scheme, and what is gained is, hopefully, some measure of caution and wisdom about the next cycle of investment.
The two viewpoints lead to diametrically opposing prescriptions for action. The first suggest the government should, at all costs, try to restore the enthusiasm of the previous boomtime, because without the return of the “animal spirits” the economy will never recover. The second suggests this is the worst possible action of the government, because it prolongs – perhaps for years and years – the period of cold-water-in-the-face readjustment of perceptions to match reality. They say you can’t cure a hangover with “hair of the dog” (more booze).
The two schools debate passionately over the unusually long recessions, e.g. this one and the Great Depression. The first says the length is caused by timidity – the government fails to act vigorously enough to counter gloom and excess pessimism. The second says the length is caused by exactly the opposite, a lack of caution – by too much ill-advised meddling by government that drags out the necessary reallocation of jobs and capital.
For example, when real estate crashes, the first school suggests government should do something to prop up prices, e.g. lower interest rates on mortgage loans, allow people to refinance even when their equity is zero (or negative), subsidize purchases through low or zero money down loans, et cetera. This will restore confidence and the market will zoom back up again.
The second school says this is nuts. They say the problem is simply that the real estate is not worth what was paid for it, and you can’t escape that fact (or rather, the longer you persist in trying to escape that fact, the worse will be the final reckoning). They say the right thing to do is allow those who are going to lose their money to lose it as fast as possible, so that they can get on with their lives, readjust expectations, do something else to restore their fortunes, having written off dumb real estate investments one hopes. Meanwhile, the asset (a house) falls to a price that is actually reasonable, and someone who should be able to buy it (but couldn’t before because of its outrageous price) gets to buy it and this time will keep it, because it isn’t beyond his means.
Economics and social psychology being as murky ill-defined non-empirical subjects as they are, I don’t think any honest person not in the grips of ideology can say for sure certain which of these is closer to the truth, if indeed either is.