I've done some homework on this and found no answer that makes sense. Hoping someone here will have some insight that I haven't been able to find elsewhere.
Every single white shirt I own turns yellow just sitting in my closet. They come out of the laundry clean, I hang them or put them away in my closet and then when I go to put them on they're yellowed. This goes for cheap white Banana Republic t shirts and my higher end Kiton, Brook Brothers etc shirts. Sometimes it's weeks or months between wearings.
I never use bleach. Actually I use no additives at all.
I wash in cold water, which gets them clean.
I line dry indoors.
It's not just the pits, it's the entire garment.
I don't think I have radioactive sweat or anything like that.
It's really annoying because it basically shortens the life of every white shirt I own and generally I take pains to take good care of my clothing and find it lasts me a long time.
Have at 'er!