IMO a combination of factors. One is that the futures rallied the last day or so anticipating a low build or a draw this week. So today's number was good but not totally out of the blue.
Another is that oil prices are down today with the terrorism plot data. Some are thinking that travel (airline anyway) will be curtailed and thus, lower demand for oil and aviation fuel. IMO this will be just a kneejerk reaction and very short lived.
In the last 3 weeks there has been -7, 19 and -12 NG build reported for a net 0 NG build over that timeframe. The five year average is 180 BCF build....IOW we have eroded a significant amount of the overhang in just 3 short weeks with more hot weather on tap.
Electric generation is up significantly during this hot spell and most must be firing with NG.
New restrictions are forthcoming in the coal mining industry in the wake of the tragedies this past year that should tighten up coal operations and quite possible drive some small and marginally operated mines out of business...and further demand for NG.
All in all, still a bullish case for NG and NG heavy operators.