It's probably due to the plastic bag.
Try storing the brass in something that's not air tight, so any moisture has a chance to escape.
Or make sure the brass is completely dry and there's no moisture trapped in the bag.
If you live in a high humidity area, leave everything in an air conditioned room long enough to ensure this.
Another reason not to reload in a hot, humid garage.
Walt Kelly, alias Pogo, sez:
“Don't take life so serious, son, it ain't nohow permanent.”