Why does a company stock price drop on good news?

I have been a broker for about 10 years. A stock drops on good news because it is following the old saying "buy the rumour, sell the fact".

The Stockmarket is all about expectations. You are always betting on a particular outcome.....sometimes it goes your way - sometimes not. Does the performance of a stock live-up to its expectation? That is why stocks move up and down...I hope that helps.