There's definitely some odd quirks with it, but I think that Tarth outpost? woke it up and now it's beelining for a city past yours. I have no idea why it does this, I've always thought it should be territorial (and to be fair, a lot of times, it is), but sometimes it'll pick a target really far away and just goes there, ignoring everything else on the way. You could toss some units or outpost up in it's path, and it'll run it over, but it won't stop until it destroyed whatever it's going after.