Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

Which shareholders cause news-driven whole market stock swings?
The people who cause this sort of sell-off immediately are mostly speculators, short-term day-traders and the like. They realize that, because of the lowered potential for earnings in the future, the companies in question won't be worth as much in the future. They will sell shares at the elevated price, including sometimes shares that they borrow for the explicit purpose of selling (short selling), until the share price is more reasonable. Now, the other question is why the companies in question won't sell for as much in the future: Even if every other company in the world looks less attractive all at once (global economic catastrophe etc) people have other options. They could just put the money in the bank, or in corporate bonds, or in mortgage bonds, or Treasury bonds, or some other low-risk instrument, or something crazy like gold. If the expected return on a stock doesn't justify the price, you're unlikely to find someone paying that price. So you don't actually need to have a huge sell-off to lower the price. You just need a sell-off that's big enough that you run out of people willing to pay elevated prices.