A crowd is distinguished by the inability and unwillingness of its members to think independently of their fellows. Instead each member of the crowd accepts the same set of "facts" which become slogans to be repeated whenever a dissenting view threatens their comfortable interpretation of events.
This is a problem because it means that the only thing that will shake the crowd's confidence in their facts and their apparent implications is a very big, very unexpected market move in the opposite direction. At that juncture the crowd disintegrates, its members retiring from the field to lick their wounds. When the smoke has cleared very few of them will have any positive investment results to show for their efforts. About all they have gained is the fun of being part of a big, like-thinking group of investors. This is what happened to the big, bullish, bubble crowd in the stock market in 1999-2000. The same fate awaits the big, bearish, the-end-is-nigh crowd in the market now.