The Shivans waltz into Capella, a colony of exiled Terrans, and blow up a star. How does Petrarch interpret this? "Oh, they must have been trying to get home, like we are."
To me, both these assumptions are flawed. The Ancients try to figure out the Shivans by thinking like Ancients. The exiled terrans try to figure out the Shivans by thinking like Terrans. This is like trying to puzzle out why the sky 'decided' to rain one day and remain clear the next.
I once sketched a campaign where the shivans' motives were to make a new wormhole into sol node, given the other was nuked by the GTVA. It would take them 40 years, but time wasn't really an issue for the old shivans. That would mean that in such a campaign, Earthlings would have been given 70+ years of peace, only to find themselves frakked by the incoming shivan armada of 50+ Sathanas.
It wasn't a good story though, the sketch was mainly interesting for the mission designs, which were to be ...ahhh... revolutionary. Of course, a revolution that never went out of my bed.