Well, since my own reason seems quite different from those listed here, I'll share...
My wife and I recently decided to let our kinkier side free. But, besides the handful of people we share those interests IRL, we can't really talk about it all. And it was a bit frustrating for me.
So I decided to vent that frustration by writing about the events that transformed us from a somewhat 'vanilla' couple (at least sexually) into our freakier selves. Once I was done, I let my wife read it. She said it was too bad noone else would read it because it had turned her on. But she feared if we shared it as is, that "someone who shouldn't" would stumble upon it and connect the dots.
Thus I rewrote it, altering everything that could let someone recognize us while keeping as true to the spirit of the actual events. She loved it but still felt a bit uneasy posting it like that. So I suggested I'd translate it to english. After all most of our friends and relations aren't fluent in english, so the chances they'd read erotica in this language was very low. She agreed.
And thus, after I was done with my translation, we looked for a place to post and found Lush...
Now we're waiting for the first of the thirteen chapters of our story to be approved (which will take some time still, since it was rejected yesterday, but I made the changes required and resubmitted).
And while I was here checking if it had been approved, I wandered into the forum, and decided to waste some time here.
There you go, you know the reason.