70%+ of the volume is big money using algo's. It WILL happen again unless the gov't puts the kobosh on this situation with all of the algos in the business which I doubt they can do effectively.
Also, why shouldn't the Dow drop 1000 points on Monday? Why not have a market consisting of a bunch of computers looking at each other? In the debt driven economy there is almost no such thing as fundamentals.
What if those guys were trying to collect rebates at 60% below the market. They turn around, get pennies on trades, only to be net short all of a sudden? How does that even work?