Perhaps it's an old enemy that has come back with a new plan. For instance, a Titan or a powerful monster that Percy thought he had defeated before. They might have found a loophole in the rules of the demigod world and managed to banish Percy to Alaska as a form of revenge and to keep him out of the way while they carry out their other evil plans.
It could be a new enemy. Maybe a powerful and vengeful demigod who is jealous of Percy's fame and power. This demigod could have found a way to use some ancient magic or some political maneuvering within the demigod world to get Percy banished to Alaska.