For example: Fannie Mae went up ~20% today, February 3, 2011. How come? What websites have you found to be useful in returning relevant information? It seems to me Yahoo Finance is not very good at this.
A few days ago they launched Fannie Mae Guaranteed Multifamily Structures (link) but who knows? It's a penny stock now.
Google Finance is pretty good at marking news right on the chart for a particular stock. That's how I tracked that piece of news down. Can't say that it precipitated a lot of people buying the stock, but Google Finance isn't a bad place to start looking.
When you look at the charts in Google Finance, they put the news on the right hand side. The time stamp for each news item is indicated with a letter in the chart. This often shows what news the market is reacting to.
In your example:
Clicking on the letter F leads to this Reuters story:
Because more people bought it than sold it. That's really all one can say. You look for news stories related to the event, but you don't really know that's what drove people to buy or sell. We're still trying to figure out the cause of the recent flash crash, for example. For the most part, I feel journalism trying to describe why the markets moved is destined to fail.
It's very complicated. Stocks can fall on above average earnings reports, and rise on dismal annual reports. I've heard a suggestion before that people "buy on the rumor, sell on the news". Which is just this side of insider trading.
At any moment, the price is where the supply (seller) and demand (buyer) intersect. This occurs fast enough you don't see it as anything other than bid/ask. What moves it? News of a new drug, device, sandwich, etc. Earning release, whether above or below expectations, or even dead-on, will often impact the price. Every night, the talking heads try to explain the day's price moves. When they can't, they often report "profit taking" for a market drop, or other similar nonsense. Some moves are simple random change.