The New York Times, citing floor trades, said new software to blame
The market outage has stretched to two hours. Official say a cyber-attack is not to blame.
"Trader on the floor of the exchange in lower Manhattan, who spoke on the condition of anonymity, said that after the suspension began, traders were told that the problem was related to updated software that was rolled out before markets opened on Wednesday.
According to the trader, the exchange said that the new software caused problems soon after trading began on Wednesday and the exchange decided to shut down trading all together to fix the problem."
Then again, if you were the victim of a cyberattack, that's exactly the kind of thing you'd say.