Two examples of abnormal high volatility -
The stockmarket crash of 1987 - Yes, the market had crashed before in 1929 but that was a long time ago. The 20% decline in stock prices that day meant that no one properly understood what was going on, nor what might happen. For example, there were plenty of rumours that the majority of banks and brokerage houses were effectively bust.
The September 11th 2001 New York attacks - Again, who knew what was happening? And how many outrageous rumours were there going around? But were the rumours so outrageous and unbelievable versus what had happened earlier in the day to the Twin Towers?
Another example of abnormal high volatility might be a civil war in an important country. What would happen to the markets if this happened in Saudi Arabia?