The article said:

Most of the time, financial markets are pretty calm, trading is orderly, and participants can buy and sell in large quantities. Whenever a crisis hits, however, the biggest players—banks, investment banks, hedge funds—rush to reduce their exposure, buyers disappear, and liquidity dries up. Where previously there were diverse views, now there is unanimity: everybody’s moving in lockstep. “The pedestrians on the bridge are like banks adjusting their stance and the movements of the bridge itself are like price changes” [...] And the process is self-reinforcing: once liquidity falls below a certain threshold, “all the elements that formed a virtuous circle to promote stability...

The article said:

Most of the time, financial markets are pretty calm, trading is orderly, and participants can buy and sell in large quantities. Whenever a crisis hits, however, the biggest players—banks, investment banks, hedge funds—rush to reduce their exposure, buyers disappear, and liquidity dries up. Where previously there were diverse views, now there is unanimity: everybody’s moving in lockstep. “The pedestrians on the bridge are like banks adjusting their stance and the movements of the bridge itself are like price changes” [...] And the process is self-reinforcing: once liquidity falls below a certain threshold, “all the elements that formed a virtuous circle to promote stability now will conspire to undermine it.” The financial markets can become highly unstable.
According to a common narrative, we have lived through a textbook instance of the madness of crowds. If this were all there was to it, we could rest more comfortably: greed can be controlled, with some difficulty, admittedly; overconfidence gets punctured; even stupid people can be educated. Unfortunately, the real causes of the crisis are much scarier and less amenable to reform: they have to do with the inner logic of an economy like ours. The root problem is what might be termed “rational irrationality”—behavior that, on the individual level, is perfectly reasonable but that, when aggregated in the marketplace, produces calamity.