What actually causes swings in stock prices?

A stock price is determined by supply/demand. Most people get that. So when the S&P500 is down 1.5% and the NASDAQ is down 3% in one day, I'm assuming that means there is a ginormous buttload of selling. Also in a hypothetical situation where nobody ever sold a share of a stock ever, the stock market would never go down. So who is actually doing most of this selling that results in this price drop? Is it hedge fund managers? Retail investors? I assumed most institutional investors that manage re