People do it when they're pregnant too.Obviously they are copying it off Instagram (search the tag #milkbath)
I think it's meant to be something to do with breastfeeding #udders #moooooooooo
My old coworker did one for her pregnancy photoshoot and when I showed them to my ex(then husband) said it looked like she was "bathing in the thing that got her pregnant in the first place". but now I can't unsee it in pregnancy ones.