I first started seeing this trend pop up in Skyrim modding, with the kinds of modded companions and bodyslide presets people were coming up with.

I just thought it was a bunch of weirdos with a strange fetish, possibly deriving from some popular branch of anime or something. But then after a few years I started seeing it randomly pop up here and there in more mainstream circles.

Based on any number of other "strange" trends that we've seen pop up or expand in just the last decade, I'd say you're probably right, that this has to be yet another weird semi-spontaneous amalgamation produced through social media.

Furry fandom alone is a classic example of how this stuff can spread like an actual contagion. Mankind was ill prepared for a globally accessible Internet.