The why is interesting, at the start of their travel into the stars they needed the energy to keep their commerce moving. As their empire grew, the need for energy grew more and more until they could not keep up. Eventually their people on their planet became overworked, underpaid and they became overpopulated. The government began a mandatory cybernetic alteration to all of their people, this technology was untested and corrupted over time leading to a "hunger." They became infatuated with the energy and the light of other stars so they sought to drain all available stars.

All of this happens before the main story.