From what I've seen, it has been somewhat gradual, but a few series can be linked to the popularity of zombies. The television program The Walking Dead has certainly exposed many of us to the undead. Movies such as Zombieland and Dawn of the Dead also contribute. Even video games have made zombies mainstream. One of the most popular modes in many Call of Duty games involves players defending against unending hordes of zombies. Speaking of video games, nice Plants Vs Zombies picture, I'm a big fan of that game : )
Basically, it seems to me to be a mostly gradual movement, with a few attributable growth spurts. I bet we humans enjoy stories about zombies because they are us. Zombies demonstrate the most primitive urges of mankind. When we look at them, we see ourselves, but in a monster's body.