Literature, TV, Cinema, you name it. They are all full of fads and trends. This is why the best books, television shows, and movies defy their eras and survive the test of time. Most of these timeless movies you can catch on TBS or TNT on Sunday afternoons, unless the NBA Playoffs, which last the better part of a year, are still on.
Dark YA literature has always been around, but perhaps it's more prevalent now. A few years ago, following the success of Sherman Alexie's own The Absolutely True Diary of a Part-Time Indian, there was a run of humorous YA books aimed at boys. Most of them were full of locker room humor. Most of them were not very good. None of them compared to Alexie's book. His story is full of much more than that. But as always, people (writers and publishers) looked to capitalize on Alexie's success. I don't have sales numbers, so I can't say whether those other books were successful. However, I read several of them and I can say that most were not very good.
Now we're in a dark trend. More accurately, Dystopiamania. We can thank The Hunger Games success for this one. I read all three of Suzanne Collins' dystopia page-turners. They were addictive and really good. Who knows how long Dystopiamania will last, but it is still currently selling to editors. The more engaging questions are, how many of these other books will be any good? how many dystopia books will teens buy before they're over the subject and moving onto something else?
Time will tell.