So why do they fight wars on an interstellar scale anyway?
The fun part is that it's not central to the story. (Except insofar as the research that leads to the problem is probably military in origin.) So I think something reasonable enough to pass but bland enough to pass off as part of the setting, not part of the story. Though it may interweave with the main plot to add a few complications.
Played with the notion of a monstrous war. There's genetic engineering in their universe, and it tends to have odd side effects because you can only code for proteins, not things; solar systems tend to have people who look alike as a side-effect. Perhaps someone accidentally engineered a race of sociopathic warriors, and generations later, they still fight wars whenever a solar system refuses to let inspection forces check out their genetic engineering. (Tends to be hard on the first generation of anyone genetically engineered a certain way.) On the other hand, that might be too dramatic a reason and dominate. On the third hand, they could hypothesize about an error in genetic engineering inspiring idiocy and their problem, and also it would provide a powerful motivation for military research.
May have to nail down their FTL and other technology, too. That really determines a great deal about war. Make it easy enough and they can fight for any old reason that we've had in history.