You know, I wish I knew! I mean, I'll be innocently reading/watching something and wham! Something about it will make my heart sing and I'm gone. For example, Star Trek--I've watched all iterations of Trek over the years, but I've never really counted it as one of my fandoms until the 2009 reboot and I suddenly found myself wanting desperately to write it in it. And I really only wrote one story there, and then the second movie jossed it, but I still want to go back and write more in the world I made.
I like Once Upon A Time (it's one of the few TV shows I make time to watch) but I don't really have a driving need to write fic in it. I can't manage to keep up with Arrow, but I am dying to write Felicity and Oliver (if I can ever manage to slot into canon. Possibly I might try for it this summer.)
Popslash was so much fun, but Bandom never quite hit that happy place in my brain. I was writing Supernatural and Hawaii Five-0 within a couple of weeks of seeing my first show. On the other hand, I went and watched Mission Impossible 4 and enjoyed it, but it wasn't until I stumbled across some awesome prompts on ghotocol_kink that I suddenly found myself writing thousaends and thousands of words. (And then everything else we blame on Renner.)
I never got into X-Men fandom, despite having seriously loved them as a teenager, possibly because I was less sure of my writing skills at that point and there was a lot of friction between comics- and movie-verse, something that the Marvel fandom as a whole seems to be more easy about (which is good, as I merge canons with reckless glee.)
I think it's maybe a combination of liking the source material a lot (I really don't have time to devote to something I don't enjoy), plus knowing I'm not going to get the stories my head wants to tell from canon (which sort of explains OUaT, as I'm in it for the Emma/Killian and until they fuck that up, I don't feel the need to go there), plus a fun, welcoming fannish community. (And that last part has eased in the past years as I feel more sure of my writing/story-telling--I'm less needy, for lack of a better word.)
All that sounds fairly calculating, when really, it's more like, 'oooooo, SHINY' and off I go. :D