Betty Liu, Executive Vice Chairman for NYSE Group, explains:

Why did trading halt momentarily on Monday, March 9?

So, on Monday, what happened was the markets opened and the S&P 500 immediately fell more than 7 percent. That triggered what we call the market wide circuit breaker, and that halts trading for 15 minutes. It allows time for market participants to assess the market conditions and make decisions accordingly. That process worked exactly as designed.