Just a guess:
The deterioration probably begins when a granule of powder comes in contact with a droplet of liquid water, particularly in a confined space like a cartridge case. (If you use your breath to blow crud out of a case, you will deposit droplets of saliva.) The granule then begins to decompose into various compounds which probably include more water. That makes the process self-propagating.
I have powder that dates back to about 1950 that seems to be good and still shoots well. It has been kept indoors, and the air here in Utah is usually pretty dry.
