A drop in the return on invested capital affects the investor. Different stocks, as well as different investment vehicles compete for investor money. Other valid reasons for selling. A deteriorating corporate financial condition. A stock's response to changing economic circumstances.
Products fall out of favor, e.g., buggy whips.
Costs of raw materials change dramatically, e.g., Rubbermaid. A company is dependent on a major customer for a substantial amount of its revenues. Wrong reasons for selling. If you've kept a stock while it plummeted, don't sell it when it has fallen so far that any remaining downside risk is minimal, compared to its upside potential. Selling by using Price Targets, e.g., automatically selling when stock has doubled or when it has dropped by a certain percentage.