Quote from Lawrence Chan:
In normal market environment, long term funds only act on market open and close time.
When liquidation is needed, however, their presence will skew the overflows in the underlying stocks, making the usual PA, price levels, etc. much less reliable.
Think about an institutional trader holding a deck of orders to sell 10 mil shares of some stock for several funds.
If you were the trader, whose bonus is based on his ability to executed the orders at better prices, you would certainly try to sell on a pop. But there is no pop today. And you have to complete the order by the end of the day ...