A speculative bubble is a social epidemic whose contagion is mediated by price movements.
News of a price increase enriches early investors, creating word-of-mouth stories about their successes, which stir envy and interest. The excitement then lures more and more people into the market, which causes prices to increase further, attracting yet more people and fueling "new era" stories, and so on, in successive feedback loops as the bubble grows.
After the bubble bursts, the same contagion fuels a precipitous collapse, as falling prices cause more and more people to exit the market, and to magnify negative stories about the economy.
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