Quote from 2gtt:
one reason is that high stress events like the April 2000 Nasdaq tank or 9/11 cause higher degrees of correlation because of margin calls. an extreme event that hammers a portfolio (especially in futures) will force selling to cover the call. usually people are overleveraged in futures accounts, and they will have to sell everything even if only one of their positions get hit.
Edward Kim
2GTT,LLC
That could be a factor. However, I also believe that this is due to the fact that markets nowadays are closely connected. You have fund managers holding positions in markets ranging from US equities, to East Asian currencies, to Scandinavian bonds. Thus, once any high stress events leads a considerable number of these "super macro" players you get either mass selling/buying of securities leading to increased correlation.
Thats one hypothesis anyways.