This happens to Apple too. It's the way the market works. It's based on speculation. Rumours are usually much more exciting than what's actually announced, people buy based on the rumours, and when the actual products are announced (even when they are good) they fail to meet crazy expectations and the market corrects - people sell.
But does that Apple analogy apply here? Apple's rumors are at a crazier scale than Google's and Chromecast in particular was barely a whisper on the street, especially with more reports pointing to Nexus 7 in the days leading up to this event.
True, I'm sure it still probably applies just to a lesser extent. I just checked and it looks like after the fall it's started to rise again. Edit: Looked again and the fall/rise is pretty much mirroring the Dow and NASDAQ today.
