I always see people saying that PewDiePie was the reason Flappy Bird became popular, but that just isn't true. By the time he made his first video of the game on January 27th 2014, Flappy Bird was already #1. It first hit #1 on January 17th and had been slowly rising as far back as October 31st 2013. Here's the lifetime rankings from AppAnnie. So I'm sure PewDiePie helped in spreading the game to even more people and maintaining its spot at the top, but it wasn't the thing that caused it to get up there. Whatever that was nobody really knows for sure. A lot of people accused them of having paid installs, but I personally believe it was just good old fashioned word of mouth on schoolyards, and that's not something you can actively duplicate.
Flappy bird is a very exceptional case. It is more general case that Google/Apple market guys look at a game among a bunch of games by chance, like it and then make it featured.
