I liked the comment about the contractor saving it for Fourth of July.
Media and government over-reaction.
Back in the day, after an artillery live-fire exercise, unused powder was burned. Artillery adjusted the trajectory of the shells both by adjusting barrel elevation and the number of bags of powder. Each canister of powder had enough bags of powder for a maximum range shot. If the particular shot didn't use all the bags (most didn't), the unused bag(s) from that canister were set aside for burning at the end of the field exercise.
At the end of the exercise, the arty boys would just lay the bags out in a long powder train and light one end. Unconfined the stuff just burns and fizzles. And, its not particulary even fast about it.
6M pounds would make a mighty impressive powder train, with tremendous amounts of smoke and heat, but that would be all. The volume of smoke would probably be toxic, so evacuating the town--after it ignites--makes sense if the wind direction is right.
But, its been sitting there building up for years and hasn't ignited. Its not like its gonna ignite just because the government and media discovered it.