In my own personal experience I will have nations declare war on me from clear across the continent that I have no way of being even remotely close to even with scouts.

I have noticed if a war is going particularly good for one side or another with my neighbors (meaning they are fighting with some other AI) then before they can really mop up, they will declare war on me and start attacking me when they haven't finished up with the other AI totally.

I think there is a point in the game that the AI says "I want to ascend" and attacks everyone around them including you.