Word of mouth.
That's why I think it happens. People read a book, love it, want to share it with others, sometimes even cramming it down their throats--figuratively, of course. Then the readership explodes. Other authors hear about it and take the genre for a ride. Do I think this is how it's always happened? No, I'm not sure, but it seems as if this is the way it happens. With Twilight, it seemed like everyone was suddenly wanting to publish stories that included as much angst as you could cram in one book and everyone was writing about some 'dangerous, slightly good, I'm-torn' creature falling for a 'plain Jane, fill-me-in-with-yourself, weak' character. The Hunger Games is the latest thing to explode, and now we see dystopian as a genre coming to the forefront.
I don't think this is necessarily a bad thing. Certainly if you love these types of stories, then it's the best thing to happen in a while, as far as books go. I think there are powerful trends in YA because kids/teenagers talk. They exchange lots of information with each other and are practically controlling the internet world, which gives them a multitude of ways to advertise the things they love. They share these things with each other, and so I think that's why you see powerful trends in YA. Adults, on the other hand, I don't think share information with each other as much as teens do. They're generally leading busier lives and have more on their plate, and I just don't think they get or make the opportunity to share books they've enjoyed.