Fear impacts our behavioural patterns, whether consciously or subconsciously. If someone threatens us with a knife, the instinctive reaction will be to take flight. If the stock market falls 20% in a single day, we will tend to react in very much the same way, even if the threat is not physical. Fear, like greed, makes people, and that would include investors, behave irrationally.
You may argue that physical threats cannot be compared to financial threats and, in some respects, they cannot, but research into the human brain suggests that it is the same part of the brain that kicks into action in both instances.