I have had an experience with brown discoloration on a couple of cotton fabrics. They were stored in my attic in plastic bags (trash type bags) one was a piece of fabric and some was leftover from cutting out a dress for my daughter. They weren't in the same bag, or box but these 2 items had some areas of brown discoloration. I was going through all my fabric especially "scraps" as I wanted to use the scraps from her dress in her quilt I'm making (there will be enouch of her dress fabric to get a patch :-)). Most of the fabrics were fine except for these 2. So it probably was the bag, being in a box could have an effect as well. So it could be just random occurance, and maybe I won't use trash bags to store fabric or out of season clothes anymore.