It sounds trite and stupid when I say it like this, but bubbles burst because there's no more money to throw at the asset class or item that has a bubble.

Said this way, most people go "Well, thanks a lot for that penetrating insight, Einstein! No duuuuuuh!" 

But it is absolutely true in every case: the thing that amplifies mere speculation into bubbles is a combination of group-think and margin or easy liquidity access. 

Examples: The Dutch Tulip craze: pretty much, people literally ran out of other assets to convert to cash to buy tulip bulbs.  Before this bubble burst, it reached a point where no bulbs were actually changing hands, but only contracts (which you could equate with leverage) were changing hands.

The Crash of '29 was kicked off by rumors that Hoover would not veto the Smoot-Hawley Tariff Act - the market "legged" down, investors got nervous, then rumors initiated another sell-off, and so on. Back then, stocks could be bought like you can buy corn futures now -- 10:1 leverage.

The dot-com bubble: the VC's funding stupid startups turned off the money spigots and stupid startups (eg, Pets.com and the stupid sock puppet) folded as they burned through all their cash, went out of business and the demand for PC's software, networking gear, etc, all turned downhill in a hurry. But while the VC's were writing the checks, you could get at least $5 million in startup capital for the most stupid of ideas. I mean, really, really, really stupid ideas. The catalyst in that case was a Barron's article that showed over 100 stupid startups that were going to burn through all their cash in the year 2000, and unless VC's were going to pony up more money, the cash from operations at that 100+ startups was nowhere near enough to cover the expenses. When 2000 came to an end, the Barron's article was proving quite correct and profit taking at the end of the year pushed tech down, down, down -- and then we were sliding into a recession as investment money was pulled out of the markets, which finished the job. Stocks in companies with no profits, no prospect of profits and no plan how to get profits could be bought at 2:1 margin. Today, you must put up 100% of the cost of the stock to buy such companies at most brokerages.

The real estate bubble is now bursting because the lenders who made increasingly stupid loans to ever more suspect borrowers started going bankrupt. House prices leveled off, which meant that some people couldn't re-fi, which crimped their ability to buy another house by leveraging off the first one, and things started turning downhill. Securitized debt instruments were being bought up by hedge funds and investment banks with 10:1 to 16:1 margin.

The event chain that finally causes real money to be withdrawn is different in each case, but it happens in stages and by the time the "true believers" are pulling their money out, the selling pressure becomes tinged with real panic. Another item that I see in studying these things is that the cautious people, the people who are eventually proven right, the people attempting to inject sane, historically justified valuations on the market -- are always early in their calls, they're always dismissed as nutjobs and scare-mongers, and they're pushed to the sidelines of public opinion by the true believers who start saying the words that everyone in any market should fear:

"It's different this time!"

When you hear that, start looking for the exits. Those words are an almost certain sign of a bubble mentality.