http://www.marketwatch.com/story/her...lly-2015-07-13

There also was a curiously high percentage of so-called self trades — when the same entity is taking both sides of the trade. The government suggested that high-frequency trading firms’ separate trading algorithms interacted with each other.

Another way to put it — there was a high percentage of the same firm’s robots betting against each other.

Self trades can be, but aren’t always, illegal. It depends if they are so-called wash sales (sic -- should be trades). The report didn’t say whether these particular trades were or not.
If you trade yourself, that's a wash trade, and illegal. Further proof that wealthy manipulators own the market.