Speaking of early game, I thought this screenshot might be appropriate... (keep in mind, the only thing I've sent off my planet by this point is scouts).
Okay, I restarted the game with the same settings, and found your problem. Those spire ships were on special forces, and unfortunately, your planet is sort of a good connector between the planet they came from and the planet they were going to.
Yes, on lower level AIs (including 7), special forces ships will try to avoid hostile planets. But they will go ahead and go through them if there is no other way or if any other way would be significantly longer. You chose a nice planet where many such pairs of planets would require a path through your home to get there within any sane number of hops.
This does raise a new issue, ships with per planet and per guard post caps (for the AI) and special forces. It seems that special forces are one way that the AI can "cheat" those limitations, and your game, poor sir, was a game the AI decided to abuse this loophole.
I don't know what can be done about this though.
Maybe prevent ships with per planet or per guard post limitations from able to be in special forces, because they have similar levels of power to starships, and starships cannot be in special forces for this very kind of reason.