Short conclusion: "In summary, quotes from NYSE began to queue, but because they were time stamped after exiting the queue, the delay was undetectable to systems processing those quotes. On 05/06/2010 the delay was enough to cause the NYSE bid to be just slightly higher than the lowest offer price from competing exchanges, but small enough that is was difficult to detect. This caused sell order flow to route to NYSE -- thus removing any buying power that existed on other exchanges. When these sell orders arrived at NYSE, the actual bid price was lower because new lower quotes were still waiting to exit a queue for dissemination."

"This situation led to orders executing against whatever buy orders existed in the NYSE designated market maker (DMM) order book."

This is actually the most cogent explanation I've yet heard.