Yes, it sounds like the pellets got moisture somehow. Powder is one of those things that can suck moisture right out of the air. Whether you can set them out somewhere and dry them I do not know. Perhaps go to http://www.hodgdon.com/ shoot them off an e-mail and see what they suggest. They might even replace them, who knows...

Just the fact you opened a new box of pellets and it fired is all it would need to convince me that something got to them....