“While our policy is not to comment on individual participation in our markets, in light of volatile market conditions, CME Group confirmed that activity by Citigroup Global Markets Inc. in CME Group stock index futures markets does not appear to be irregular or unusual in light of market activity today.”
-zero hedge

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Ummm, yeah. No kidding.

Thin market, negative feedback loop.  That is what happened, not a mashing of keys from even the chubbiest of guido fingers.