Speculation occurs when people buy assets expecting their prices to rise. Presently the reality is not what the asset will earn in the future, it is only that enough people are expecting the speculative object to rise in price to make it advance in price, and thus attract yet more people to yet further fulfill expectations of yet further increases.

The process has a pristine simplicity: it can last only so long as prices are reliably rising... Whatever the pace of the preceding buildup, the resulting fall is always abrupt. John Kenneth Galbraith, Money (1975)