I agree with the commenter who suspects manipulation. The mkt’s acting like there’s a big, aggressive seller: there are size bids, but only small resting offers below about 53. The seller is the only one tightening the mkt. Also, IIRC (and I may not be), it appears there’s been huge volume in the past hour or so.
This behavior doesn’t quite fit the pattern that observers noticed in the mkt earlier, where someone came in when the mkt was thin and spiked it down quickly by taking out a lot of bids. Today’s price action looks more like the scenario I hypothesized about, where someone is willing to spend hundreds of thousands to hold the mkt down, for an extended period, by entering a stream of sell orders in small units. Not as dramatic as the big plunge, but perhaps more believable to uncommitted or easily-discouraged voters. (And anyway, there are probably now so many resting bids at lowball prices that the mkt can no longer be spiked.)
If I were George Soros I’d do the same thing.
(Related post here)
UPDATE: Don Luskin shares another theory.