The reason the fairies burst from the Earth was because they needed to escape the Plague that was spreading beneath the surface. They didn't tell the humans this, mostly because if they did, the humans would probably have kicked them out to make sure the Plague didn't spread to THEM.
Well, only those who were screened not to have the Plague were let out onto the surface anyway, so nobody thought anything bad would happen. But it turned out ONE infected talking bunny had escaped the radar of the border patrol, so thus DOOOOOM was unleashed out upon the world. March 25 was the day that the first human on Earth, a Mexican guy named George (don't ask), saw the cute little bunny, went "AWWWW COSA ES LINDO!" (translation: "AWWWW THAT THING IS CUTE!"), and pet it. Of course, this meant that he had now caught the Plague and doomed most of humanity, but hey -- that thing WAS pretty cute.