Imagine you have a share in a company. How much would you, personally, sell that share for? Well, it's a function of what it'll pay you in dividends over some time horizon. Maybe you're sure it'll pay about a dollar this year, and it seems likely it'll pay a dollar fifty next year, and you're not really sure after that but probably not nothing. So you want at least $2 for it, but you figure in the most likely case is that it'll continue to pay out and wouldnt sell it for less than $10.

Now, suppose some news comes out that profits are forecast to be half what you thought, so your estimation might come out at $5. Except now you're also worried about two other things: first, the company is now more likely to go under before you can get your $5. And also, before you were quite happy that if things went badly, you actually could sell your share for about $10. But now lots of people are trying to get rid of them at once, so you're worried it won't be possible, so you decide actually you're willing to sell at a lower price.