Presumably most 401(k) contributions and dividend reinvestments happening automatically create a large volume of buying power.
While some people freak out, call an agent, sell stocks, I'd imagine this is mostly actively managed (by humans and software) investments doing the selling.
Beyond that, when you sell something, maybe it felt like it was worth $10. But you offer it for $9.95. Maybe someone buys it, maybe not. Maybe you drop it to $9.80? Then someone buys it. Rinse and repeat until the price is now $8.
When you sell a "$10" stock, if you say you won't sell it for less than $10, you might not be able to do so. It's all more complicated than that, and there are limit and market orders, etc. that affect how much the price will vary from when you initiate it.
But if you're like "hmm, this is horrible news for the economy" you probably press the "sell as fast as possible" order, and your $10 stock might actually sell instantly for $9.50 because for someone with a "smart" algorithm, $9.50 looks pretty good when the stock still seems valued at $9.75+. But then a few million shares sell at prices lower than $9.75 and now the same system that thought it was a good idea to buy at $9.50 thinks it's a good idea to turn around and sell it... for even less.