At the end (or i save/quit for a brake) of some single player games, i see that one AI had just steam rolled another AI with amazing speed. Even if they are attacking you with a huge fleet, another AI could be muching away at their territory until all they have left is that big fleet they amassed one jump away to crush your front line world (happened to me once, BIG ass fleet just sitting one jump away, i'm all worried he might break through, and then i notice i can no longer place bounty on him because he's dead).

They send in scouts often, and if they see you only have static defenses he'll attack. Even if that scout makes it to the next system where your huge assault fleet is assembling for the destruction of his empire, he'll assume that you won't notice and send in a attack fleet anyway. Sometimes he'll turn around and just run once he sees your defenses up close (happens a lot with siege rushes i find).

So to help discourage attacks you could leave a moderate size fleet with the static defenses and the AI will be less likely to attack (just make sure you have 2 or more repair bays in case he goes for it). I personally like to let him attack and then make a rush for his capitals (you need to slow his jump out of the gravity well for it to work).

Hahaha, I think it's because of:
1) You're awe-inspiringly powerful
2) You're the most convenient target

or, if you just so happen to:
3) have annoyingly colonized a world, right next to their homeworld, that they only just cleared of pirates and were about to bring a colony ship in when you appeared with a colonizer capital ship.