My thoughts on the situation is somehow Amon is the manifestation of Korra's spirit world connection, since Korra has no connection herself. Through Aang's loss at the last epiode of TLA Season 2, where Aang was shot down by Azula, the Spirit connection has been severed. The water from the spirit pond in the North Pole may only have saved Aang's life, but jacked up the Avatar Cycle somehow.