Alright, I made a fairly good map this evening, 3 main star systems, one player starts in his own system.  roughly 33 planets per system, and they are spiral towards their sun.

Anyway... so I'm trying out my custom map, doing pretty well (holy crap trade income gets high when you have a 32 phase trade route).  Amassing a force to start my first offensive on one of my two opponents, when all of a sudden I get the message I have lost a planet to enemy forces, and im sitting here thinking "WTF?", considering I have a starbase on each planet 9 jumps away from the star (its a safety blanket).  Then I find out I am being attacked my neutral "rebel" forces.  These guys are jumping into my planets' gravity wells from thin space!  And to make things worse, they jump in not very far away, so when I do get a message of their coming, I don't have time to fortify my planet against these rebels.

And what is really confusing me is that I have never seen this before.  All of my previous custom maps, of similar size, haven't ever had this problem.  And I've never even seen it before on a classic map.

Anyone know if this is a bug, possibly because of how long my trade route is, etc?  Any insight here would be helpful, I'd prefer not to have to fortify every single planet past my "security blanket" of starbases, mostly because of the sheer cost and time I could be investing in forwarding my attacks.

Thanks!