It probably stems from Japanese porn, but it's the fetish that translates the best to real life imo. I always found sniffing to be a turnoff in porn, but I can see the appeal to the smell of a women's armpits. It's like a personal scent, and the the most tolerable and least gross of all the body odor to me.
It's not something I seek out, but with certain girls like Emily or Dizzykitten it's just a part of the body I find particularly attractive on them. Like how you find certain features attractive on different girls. I know I make posts of both, but with Emily in particular it just seems to annoy people in the thread, so I also do it because that's funny to me. Also, her thread would be more dead without me, so...
I also know it's kinda below feet on the judgemental fetish scale.