It should't. I'd love to check out your save game. There are so many lines of code that literally focus on making sure that the AI can get to you before it does this.
Starting in 1.4, the AI does assemble battle fleets in advance of a war. It's a new new system that will continue to be expanded on.
I don't know how useful my save is to you at this point since I have kept on playing. The AI did actually send a fleet to attack me after a while, but only a starbase that I constructed near their border (which is ironic since I was planning on using it as a springboard for my own invasion, only to get completely and utterly creamed by the sudden attack). After they blew up my starbase and crushed the fleets I had assembled there, I don't know what has happened to their ships. It appeared as if they were on their way towards my systems but I haven't seen them so far, and suspect that they simply retreated back into their own regions of space.
On the bright side, the fact that the AI demolished my fleets isn't something that has happened before. Though I suspect this may be largely because of my experimental fighter design which did not work out so well. It was a small hull with 1 railgun and 1 rocket. Their ships had point defence however and completely WRECKED my ships. I had to send like 3 or 4 fleets just to kill 1 of theirs. Needless to say I upgraded my designs afterward.
