The point is that when novice investors finally know about it and get in, all the good investors have already been in for a while. At that point, there is no one left to buy. If there is no one left to buy, the stock quits going up. Of course then people will start selling, the smart ones first.
If it was always at equilibrium, price would always be the same. A crash occurs when there are no buyers. To generate demand people offer their shares at very low prices. Then hopefully someone will buy. But if no one does, the offer their shares at even lower prices, and that's how the tumble works.
