I'm going to have to back up a moment and talk about the self-publishing industry, because while I think a lot of people on here are familiar with the concept of self-publishing, this part of the internet doesn't have much contact with the industry of self-publishing. The part where the big money is.
Mostly, that happens on Amazon, and involves books that look like this:
(this is just a selection from the current top 100 in LGBT romance on Amazon. I'm not familiar with any of these books in particular and the following may not describe them specifically, this is just to capture the general look of the thing.)
It will be impossible for me to cover all the dynamics at play in Amazon self-publishing, suffice it to say that there is a lot of money to be made hitting the top of the Kindle store. That wolf book up there is currently at #971 on Amazon, meaning it's selling around 114 copies a day, which at its $4.99 pricepoint is $568 a day, which is $17065 a month. The author certainly has other popular titles as well. This is oversimplifying, but you get the idea. There’s potentially a lot of money to be made.
The broad rules of the game are to release books as often as you possibly can (some authors release entire novels monthly) and to write books that copy established trends and dynamics as closely as possible (remember this, it will come up later.)
That means trends will rise and fall quickly on the charts, since a lot of the biggest players release books on a rotating 30-90 day basis. An mpreg book starts selling like crazy? You're guaranteed to see dozens of others within the month. It's a grueling race to keep up with, and as you can imagine AI has thrown a wrench in the works (but that's outside the scope of this post.)
In 2010, the heyday of the Twilight fandom, none of this had happened yet. Ebooks had just barely begun to become a cultural force. Smartphones (which were new at the time) meant that suddenly everyone was carrying around an e-reader with them 24/7, meaning ebooks were suddenly much easier for people to pick up on the fly. And unlike with a physical book, when you're reading an ebook, nobody can see what it is.
Enter Fifty Shades of Grey.