[sector24]

It's based on the AI's belief that it can successfully take a province if it declares war on you. For instance, you can heavily defend all your provinces except 1 with no PD and eventually the AI will look at your army graph and decide you're weak. It'll build up what it thinks are a sufficient number of troops and attack you.

Now if you have more than 1 "open" province it just makes the AI attack you faster. It seems like the AI is always at war with at least 1 nation in order to expand, so if all of its neighbors appear tougher than you it's just a matter of time before they attack you.

Further, the reason you get ganged up on is because one nation declares war on you and you expend resources to defeat them, gather more lightly defended provinces while at the same time losing troops. Another nation attacking you just increases the number of lightly defended provinces and lost units, encouraging more opponents to gang up on you.

That's my take on it anyway. I've never had a border nation attack me when they had nothing to gain. Occasionally they declare war on you via a message but they're always ready to take something the next turn.