Today's date is the 25th anniversary of Black Monday. A well known and severe crash. There were four other well known severe crashes that occurred in the 20th and 21st centuries, that did not occur as a result of a sudden external event. (I.E. WWI WWI, The crash of 2001 was in response to 9-11, the flash crash of 2010 was the result of a computer glitch).
The years for the five best known and most severe crashes, of the 20th and 21st century, that were not the result of a sudden external event were:
1987 (Black Monday) October 19
Very interesting "commonality" indeed.