Nothing fundamentally has changed [recently]. The psychologically has changed. That's all. People have to understand that markets get driven by fundamentals in the broad term and they get driven by psychological's in the short term. The psychology simply changes and that's all that's happened here.
Has the Economy suddenly changed overnight to manifest in a sharp decline in stock prices.. No....never happens that way. Its just the psychology that changes, you have margin accounts that were terribly high, probably the highest level, aggressively extended as they have been in many many years and that sort of things happen. Risk happens fast, economic activities change much much slower.