Selling after the market falls
The short-term direction of the stock market is unpredictable; yet selling in reaction to market moves implies that you can predict short-term moves. The markets are not perfectly efficient from minute to minute, but they quickly reflect a best guess based on new information. Fear is one of the greatest enemies of successful investing. When you're worried about your money, you want to make it safe. However, you risk missing out on the next rally, and you might not even keep pace with inflation. From a long-term perspective, cash is very risky and stocks are low risk. Put another way, this is another example of selling low and buying high. Savvy investors go bargain hunting when the market is oversold; you should, too.