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.