1. There are always buy orders listed. When sellers is rushing to unload
their stocks, they take whatever offer is on the market. More and more lower
buy orders get filled and the stock price is going down.
2. Vice versa, the institutions start buying shares whenever available and
the stock price go up. Once the up trend is clear, small investors starts
jumping and drive up the stock price and that is when the institions start
unloading their shares.