Housing bubbles usually start with an increase in demand (a shift to the right in the demand curve), in the face of limited supply which takes a relatively long period of time to replenish and increase. Speculators enter the market, believing that profits can be made through short-term buying and selling. This further drives demand. At some point, demand decreases (a shift to the left in the demand curve), or stagnates at the same time supply increases, resulting in a sharp drop in prices - and the bubble bursts.