Fictional characters don't need a reason to be trans any more than real people do. Just as cis people evoke their own experiences in what they create, so do trans people. If it suddenly looks there's a lot of trans fiction out there? Oops, guess there are a lot more trans people in the world than anyone thought. If it bothers you, well, hopefully our dev team will get around to implement tag exclusion soon. We know it's a priority. Promise.
Oh and by the way, Tumblr-bashing is just boring by now.