Well, the actual problem is that there are two overlapping markets, they just happen to buy the same stocks.
There's individuals like you and me who buy companies with our own money based on how we view the future of those companies. Based on the people I talk to I don't think people like us have done much over the last 4 years. Everyone I know just keeps holding onto their shares.
If we were all there was, it would be a very stable stock.
But we're overlaid with the other kind of investor: The fund manager. This is a guy responsible for billions of dollars of other people's money. Like, say, a pension manager for a city government or large company. It's not his job to get a huge return, but rather to hit a specific (but safe) annual return.
Those are the guys that pumped a ton of money into Apple and shot it up to $700. They're also the guys who moved on at that point because it wasn't going to go much higher. And that was the right move for those guys. They didn't do anything wrong. It's not their job to invest in companies long term, it's their job to protect their own business' money.
But what that does is rock the ocean for you and me. We own sailboats and they're oil tankers. And we're all playing in the same pool. And when they stand up it has a big effect on the water level.