Heat, ammonia off gassing, the quality / manufacture of the cellophane itself.
Years ago, I put a bunch of random empty cello sleeves, including some that had never been used on a cigar, into some tupperware containers. One container had a small ramekin of ammonia, another I left in a warm place, last was one with both ammonia and heat. Left them sealed for a few months, they call came out yellowed/browned to some degree. The heat & ammonia container being the most yellowed. Based on texture and thickness, it was pretty obvious there were different manufacturers or different formulas / specs between them. Within every container there was some uniformity with the cellos that came from the same source, but there was variation in the yellowing from one manufacturer's cello sleeve to another.
