First, harvesting tax losses is popular enough that it can have a marked impact on a stock's price. If a stock has fallen substantially during the year, those losses often get worse during the fourth quarter as tax-loss sellers dump their shares for whatever the market is willing to pay. Moreover, institutional investors like mutual fund companies often have a different fiscal year, forcing them to consider tax-loss selling by the end of October rather than waiting until the end of the calendar year. Selling early can give you better prices for your stock losers than waiting.