According to the academic world view that markets are efficient, only the revelation of a dramatic piece of information can cause a crash, yet in reality even the most thorough post-mortem analyses are typically inconclusive as to what this piece of information might have been. For traders and investors, the fear of a crash is a perpetual source of stress, and the onset of the event itself always ruins the lives of some of them.

Chapter 1, Financial Crashes: What, How, Why, And When?, p. 3. - Why Stock Markets Crash - Critical Events in Complex Systems (2003)