If there is a lot of demand for a stock, investors will buy shares more quickly than sellers want to get rid of them. This can move the price higher. On the other hand, if more investors are selling a stock than buying, the market price will drop.
There are plenty of catalysts that can push the market up or down. For example, a market can fall due to things like inflation pressures, supply chain issues, rising interest rates, and recession fears. These factors resulted in more investors selling stock than buying, leading to more investors selling and possibly even a stock market crash!