I lactate and I'm not pregnant. I just never stopped producing milk. It's not completely unheard of. If you suck just the right way it produces the stimulation my boobs need to give out a pretty good amount of milk.
I enjoy those sessions with hobbyists because I've been told that my milk is the sweetest they've ever tasted.
It's definitely something you have to really or be interested in, or of course you'd think it's weird.