Bleaching whites and sunlight

So why is it that when I’ve soaked my white knickers/tops/boxers in a bleach (sodium hypochlorite) solution, and despite repeated rinsings, when I hang them on the line they invariably turn a brilliant shade of purple.

This is only a temporary phenomenon (thank goodness) and they rapidly return to their original whiteness, but I still wonder why they turn purple in sunlight.

Any musings on the matter here?
(And don’t give me complicated chemical formulae orright!!)