Quote from clearinghouse:
This is my guess on what happened, based on Nanex's report:
When you try to put an order in for a midpoint peg, for example, you have to send your buy order at the asking price and your sell order at the bid price. If your midpoint peg flags aren't "on", the order will actually cross the spread.
From what I can gather, they were trying to do their NYSE retail liquidity program which allowed for sub-penny pricing on the NYSE.
So my theory is that some order flag or some such wasn't set, and this caused them to buy at the ask and sell at the bid over and over, cycling through volume rapidly, because that program might have worked similarly to the way midpoint orders work. The program must not have realized they weren't being pegged and just went rapidly out of control across the board before they had some way of turning it off.
That's my guess anyway. I guess the real question is why they didn't turn it off as soon as they knew something was screwy. That's the big question.