The analogy I would use is that investors may know something is going to happen, but there is a lag before it really happens (less market share), and the stock price decline lags that. In school they teach you that a stock tracks the future predictions but it doesn't quite work like that. For big, widely held things, the stock often lags at predicting future troubles until the last group of investors realize that the trouble exists. That also explains why when stocks fall they often fall quickly rather than gradually.