Bizarre robot stock traders

Updates: see also How Algorithmic Trading Works, and Explaining Bizarre Robot Stock Trader Behavior.

While The Atlantic does have some editorial problems, see for example Atlantic shocker: Senior editor Clive Crook fabricates another quote to smear Michael Mann, here's an excerpt from Market Data Firm Spots the Tracks of Bizarre Robot Traders by Alexis Madrigal:

"Mysterious and possibly nefarious trading algorithms are operating every minute of every day in the nation's stock exchanges.

What they do doesn't show up in Google Finance, let alone in the pages of the Wall Street Journal. No one really knows how they operate or why. But over the past few weeks, Nanex, a data services firm has dragged some of the odder algorithm specimens into the light.

The trading bots visualized in the stock charts in this story aren't doing anything that could be construed to help the market. Unknown entities for unknown reasons are sending thousands of orders a second through the electronic stock exchanges with no intent to actually trade. Often, the buy or sell prices that they are offering are so far from the market price that there's no way they'd ever be part of a trade. The bots sketch out odd patterns with their orders, leaving patterns in the data that are largely invisible to market participants.

In fact, it's hard to figure out exactly what they're up to or gauge their impact. Are they doing something illicit? If so, what? Or do the patterns emerge spontaneously, a kind of mechanical accident? If so, why? No matter what the answers to these questions turn out to be, we're witnessing a market phenomenon that is not easily explained. And it's really bizarre."