We shouldn't be so sanguine about taxes and impediments to high-frequency trading if we are upset when high-frequency traders leave the market. Those are incompatible ideas.

    As a side point: traders have stop loss levels; one big move triggers other moves. There are systematic, discretionary, and plain-old panic trades.

    But for all of those styles and programs, once they see the stock market fall 6%, a liquidation effect takes hold. That's just a function of people. Someone screams fire, and if enough people start running, everyone will. Those are the dynamics of computer software, people, animals, fires, whatever. It's how we work. That kind of stampeding effect could easily be part of the response.

    But the speed of the market falling down, going back up, and partway back down again? If this was really a stampede, why not repeat the 1987 crash [which kept going]? Nothing 'stopped' this crash except that the catalyst seemed to have ended.

    If it was an error or a software bug, it stopped. If it was a hack, the hackers left. In other words, the enabling side of this drop is totally irrelevant [to the catalyst]. The only interesting thing here is the catalyst. If this was a gas pedal that was stuck, it would've looked differently, kept going.

    Whether this was intentional or unintentional, it happened all at once. If it was an intentional [attack], then the question is, was it a demonstration, a test, or the attack itself? Whatever it was, we didn't stop it. It stopped itself.