Hmm, I might have to tap Aviy for further detail, but by going with what we have on our god docs, it works in the way of 'power stores'. Towards the end, nature gods had really big power stores because the last of civilization was praying for them to stop, shocked into believing in them again, etc. Gods on the other side, who originally had a lot of power from before the war, were using that power to help people survive and it kind of dwindled as nature gods built up. Our general idea was that when the last humans died, some gods still had power that they hadn't used up to that point. They waited for the nature gods to stop paying attention to them, slowly losing power as they waited, then combined their residual power in a last ditch effort to start pulling people.

There may be some holes in this though that I can't see, so if anyone wants to stick their fingers in them it'd really help us out in figuring out how to better gear the gods.