Nothing! Natural aging...hasn't even been worn in the sunlight that I remember.
I wear it around the house at night often while watching TV to help break it in/soften it up. I think the couch material rubs off on the creases some and make those nice looking dark lines near the elbows. Sized up to XL like I normally do w/ IH. I put some 25 lbs dumbbells in the sleeves near the elbows because they were so tight/stiff at first. Then I hung the shirt. Worked like a charm. The shoulders are also a little tight, but are loosening/softening up. Fits like a glove now and is still molding to my body some I think. I have worn it out and about probably 4 times, usually at night.