I had this theory that the Shivans are from a different 'dimension', for want of a better term, and that being aliens, we can assume they think differently.
I also figured they are attracted to subspace disturbances, in the way Sandworms in Dune are attracted to vibrations caused by people crossing the desert.
In the beginning of the GTA, these subspace 'vibrations' would not be enough to attract them, but once lots of systems start being colonised and subspace travel became more common, the increasing strength of the vibrations would have been noticed by them.
As for Capella - Clearly they are harnessing something about it - Maybe it's energy? - To transport themselves back to their own dimension, or something along those lines; Why else would they sacrifice half their Sathanas fleet to do so?
If they just wanted to trash the Capella system, they would have had more than enough Sathanas ships to do so and would have likely lost a lot less ships.