There are probably two stories regarding why this happened — one about fundamentals, and another about finance. In fact, this is typical for bubbles and crashes, not just in stocks but in every asset class. There's almost always some kind of connection to fundamentals — some story about how we're in a new economy, followed by doubts about whether that story is really true, and so on. But the big market movements are almost always accelerated by purely financial factors — "noise traders" armed with piles of excess cash, opportunistic speculators looking to ride the wave of sentiment, and so on.
