(Tumblr ate your ask but AOL still had it!)
Hehe, it can be quite confusing!
Well, it started when the Hetalians and Homestuckers started doing those ‘Dear Homestuck/Hetalia’ posts and people thought it was cute, and because neither fandoms having good reputations and was constantly put down. Along with that, the two fandoms have quite a past, where they used to hate each other before realizing they weren’t so different. From there it was pretty much decided they’d be Moirails(or Platonic Soul Mates if you’re not in HS) since both knew what it was like to have such bad reps, but not really deserve them anymore.
Then, what REALLY started all this was when someone made detailed Fandom characters designs for Homestuck and Hetalia in the Homestuck style. People jokingly brought in the Big Three(Superwholock), but joking became people who seriously just wanted to be friends, and it went from there. It continued to go through Tumblr, with people adding their own fandoms one by one with people just trying to have fun.
And turned into what we have now.
In truth, I think it was just that the quiet kindness got louder then the shouting haters. :3
I hope that helped! ^_^