It didn't appear as much as it was broken, in an attempt to free Lucifer, the Devil, from the cage he was trapped in by the angels. It seems the Seal had an anti-tamper resource and, as a result of the impending Apocalypse, it started drawing in people from all over place, to fight what was happening, presumably.
Sam and Dean Winchester would know more but I can't find anything in the literature of this world about repairing broken Seals. As far as I can tell, once they're broken, they're broken.