The short answer is that anyone who knows the details isn’t telling.

what happened

Last Thursday, May 6th, was a generally bad day on Wall Street.  What makes this session worthy of not, however, is the Tower of Terror-like plunge that the market took at about 2:45 pm, followed by an equally swift rebound several minutes later.

visiting the SEC

Representatives of the major exchanges met with the SEC yesterday to discuss the situation.  All reportedly professed to have no idea what caused the sudden decline and rebound.  That’s not really much of a surprise.  On the one hand, no one wants to take the risk of becoming Congress’s latest whipping boy if they admit to having had even an innocent hand in the debacle.  On the other, its track record in cases like Madoff suggest that the SEC doesn’t have the knowledge or interest to pursue the issue by itself.  So anyone involved would likely be calculating that, absent a public declaration of repsonsibility, they will never be found out.

rumors

There have been two persistent stories about the trigger for the mid-afternoon drop.  One, offered by CNBC, is that a trader from Citigroup made an input error that generated a sell order that was 1000x the amount intended.  Citi denies this.  The second, more detailed–and therefore, I think, initially more plausible–comes from the Wall Street Journal. According to the newspaper, the initial trigger was a relatively small sell order placed in the Chicago options market by hedge fund Universa Investments.   However, the Journal may have been attracted to this trade as much by the fact that Universa is linked to Nassim Taleb, who popularized the idea that disruptive “black swan” events are much more numerous than academic theories allow for.

what we do know

Several things are clear:

–as the computer-to-computer selling got more intense, the NYSE turned its machines off and reverted to manual processing of trades.  Apparently, although I wasn’t aware that this was the case, this is the NYSE’s publicly stated policy.  This action effectively eliminated one of the main ways derivative holders could hedge their positions with offsetting exposure in physical stocks.  It caused trades to be redirected to other middlemen, who buckled under the added pressure.  It isn’t clear whether the others took defensive measures similar to the NYSE’s or whether they were simply overwhelmed by volume.

–ETFs were hurt unusually badly in the selloff.  According to the Financial Times, two-thirds of the securities where exchanges subsequently cancelled trades were ETFs.  ETFs themselves represent only about 16% of the total number of securities traded on US exchanges.  I don’t think there’s any great significance to this, however.  In a “normal” stock trade, the market makers matches a third-party buyer and seller–who have already decided they want to transact–and charges a bid-asked spread.  In an ETF trade, however, the counterparty is often the market maker himself, who is constantly calculating the NASV of the underlying ETF and his hedging possibilities and deciding whether to transact at a given price or not.  To let the market know he is potentially a buyer or seller, he typically has “placeholder” bids of, say, $.01, that under normal circumstances let the world know that he may be interested in transacting.

Last Thursday, a lot of those $.01 bids were hit before the market makers could withdraw them.

–as mentioned above, the exchanges unilaterally decided to cancel transactions they considered to be anomalies.  This presumably leaves a lot of unhappy individuals who had placed low-ball limit orders.  Perhaps not by coincidence, the counterparties–that is, the losing side–to those canceled trades are the same exchange members who canceled them.

what happens next

The SEC is under political pressure to do something to prevent a recurrence of anything like the near-instantaneous decline of 5% that happened last Thursday.  Some of this pressure likely comes from proponents of having a stronger “uptick rule” (see my post on this topic).  Although the current uptick rule seems to me to invite sudden plunges in stock prices of the type we had last week, no one is publicly suggesting that this is the cause of last Thursday’s meltdown.

Whether needed or not, a new trading halt mechanism is probably the “solution” the SEC will opt for.

I have two related thoughts:

–added volatility may just be a fact of life in a modern stock market, where computer-to-computer trading and monthly performance measurement of the type hedge funds undergo are prevalent.  In other words, get used to this.

–the first time something like this happens, it’s a surprise.  Even now, I’m sure that investors are beginning to devise strategies to cope with–and take advantage of–increased volatility.  Understanding what’s going on and planning for it, investors will enter the market as buyers sooner and more aggressively. By doing so, they will temper volatility.  For their part, sellers, learning that there’s a penalty for being aggressive on the downside, will adjust their behavior as well.

That’s why I think canceling money-losing trades made by people pushing the market down is a really bad idea.

–no one so far is questioning the behavior of the NYSE (admittedly, not my favorite organization).  It’s kind of like switching from big fire hoses to small ones as soon as the fire starts.  What good is that?  In fact, if this incident isn’t the accident that I think it was, short-sellers may have been counting on the NYSE to remove liquidity from the market.