i would say a huge pulse of magma is feeding up into the magma plume under Yellowstone/the west of the usa, and as it does so, gases are being released, and escaping through the faults that created the peaks of the rockies

when this extremely hot material reaches the magma chamber(s) under yellowstone, the fun may begin. could be years away, BUT the Chaiten eruption 8 or so years ago, had a magma feed from deep that rose at WALKING speed, which is jet speed for deep magma

how long would it take you to walk from the west coast to yellowstone?