I’ve talked before about how hot it gets when you record. Not just vocals but other things too. Maybe a surge of adrenaline or something else, I don’t know (honestly, oxytocin is a hormone released during singing but it’s supposed to calm you). Here’s the picture you’ve been waiting for. Boobies! More accurately; bra-less boobies. So the sweat doesn’t get soaked up by the cloth in the bra, but ends up in a nice rorcharch on your shirt/top.