I don't suppose anyone knows any further details beyond this
An error message that was duplicated millions of times overwhelmed network routers at the New York Stock Exchange Wednesday, leading to a four-minute halt in trading just before the closing bell.
Trading resumed normally at 9:30 a.m. Thursday.
In a statement, the NYSE blamed the system failure, which suspended trading at 3:56 p.m., on a communication problem. In an interview Thursday with cable-news channel CNBC, NYSE CEO John Thain said the problem was caused by a "network storm" in which "an error message was created and then duplicated millions of times," overwhelming both the system's primary and backup network routers.