Nasdaq’s internal review of the hours-long trading shutdown of August 22, 2013 seems to show that its communications gear was overloaded. Nasdaq said that it is working on the issue and will strengthen its systems.
Apparently, a flood of messages from NYSE Arca hit the Nasdaq system at a rate of over 2.5 times what the Nasdaq system was designed to handle. Its capacity exceeded, the system degraded.
Contrary to some reports (or guesses), high frequency trading played no role in the shutdown.