J.Lo Hints at Divorce... Stock Market Reacts!

Why do news organizations constantly report some random news story then follow it by saying "the stock market reacted to the news in such and such a way" - as if the two events are connected?  Who decides what news causes the stock market to react?  If the Obama administration decides to have a press conference on debt, does that cause a bunch of random traders to go "I need to buy some Dell stock"?  How can anyone say the entire market reacted to some news story?  Who decides such things?  How do they know it's not something as random as J.Lo hinting at divorce?  I mean really.