I know some wikiers have been posting Book 2 predictions/theories so Im going to post my villian theory. I believe the spirits are acting up due to one person and not something like modernization or less spirituality. First off they are only attacking the Southern Water Tribe. If modernization or lack of spirituality was their problem they would probably attack Republic City first. There are two people I beieve could be a human villian in Book 2.
Varrick: He is rumored to deal with underworld spirit charcters and gangsters to make a financial gain. In Rebel Spirit a spirit attack a boat and Unalaq said spirits were attacking other boats also. Varrick is in the shipping industry. He could have sent the evil spirits to attack his competiters ships so customers would be forced to use his company for shipping.
Unalaq: Unalaq has a very strong spiritual connection. If he is able to send spirits away I bet he can send them to the human world also. He says spirits are attacking the Southern Water Tribe because they are not spiritual any more. I think this is a plot for him to become chief of both tribes. He'll need reasons to rule both tribes and saying he could stop the spirit attacks would make people want him to become their chief.