Sometimes companies have one business that's doing quite well and another that's a real stinker. Shareholders may want to own the good business separately and let the market value that without the drag and risk of having the stinker weighing down on it. In the end the stinker is either dismantled, taken into bankruptcy or sold off.

In Yahoo's case it's a bit unique in that the only thing they really have that the market likes is their holdings in other companies. Essentially Yahoo is like a mutual fund that made some good investments but the fund managers are blowing it by running a poorly performing lemonade stand on the side. A lot of the investors just want the holdings and then are happy to let the stinker, Yahoo's original business, just go away. At the moment the market assigns the core business a negative valuation so it's beyond a stinker, it's an anti-company offsetting the value of investment holdings elsewhere under the corporation. It's a real mess.