Crashes, corrections and bear markets cannot be predicted exactly

Nobody can predict exactly when a bear market will arrive (although there's no shortage of Wall Stree types who claim to be skilled fortune tellers in this regard).  But when one does arrive, and the prices of 9 out of 10 stocks drop in unison, many investors naturally get scared.

They hear the TV newscasters using words like "disaster" and "calamity" to describe the situation, and they begin to worry that stock prices will hurtle toward zero and their investment will be wiped out.  They decide to rescue what's left of their money by putting their stocks up for sale, even at a loss.  They tell themselves that getting something back is better than getting nothing back.

It is at this point that large crowds of people suddenly become short-term investors, in spite of their claims about being long-term investors.  

• They let their emotions get the better of them, and they forget the reason they bought stocks in the first place - to own shares in good companies. 
• They go into a panic because stock prices are low, and instead of waiting for the prices to come back, they sell at these low prices. 
• Nobody forces them to do this, but they volunteer to lose money.

Without realising it, they've fallen into the trap of trying to time the market.  If you told them they were "market timers" they'd deny it, but anybody who sells stocks because the market is up or down is a market timer for sure.

A market timer tries to predict the short-term zigs and zags in stock prices, hoping to get out with a quick profit.  Few people can make money at this, and nobody has come up with a foolproof method.