Why do the armpits of my white undershirts turn yellow after they go through the laundry?

It's because you use an antiperspirant containing Aluminium zirconium tetrachlorohydrex gly.

"When mixed with sweat, aluminium zirconium tetrachlorohydrex gly is known to stain clothing with a yellowish tint. It can also cause a stiffening of the affected areas of clothing. These stains can be removed with the application of vinegar or a mild bleach."