I don't know where this idea that governments declare war without a reason come from. Even Nazi-Germany and the Soviet Union always had some flimsy excuse for their invasions. Nazis claimed that Poles had crossed their border and thus the invasion was purely defensive. Commies invaded Afghanistan with the pretext that the local Communist Party invited them in to help stabilize the country - of course after manufacturing the coup of the said Communist Party in the first place.
Yup, even declarations of "right of conquest" are usually made after a pretext is brought up. The plan for outright annexation by that right is made after the start of the war, after the target has "ask for it" and invited such a punishment.
Whatever it is about us, we demand reasons for things like wars and they're rarely purely aggressive, never "I'm invading you because I feel like having more land, lol".
There is no reason that should be the case with space aliens, or human-alien relations, unless these relations were rock solid and well established.
And that's why I feel Paradox is out of their element here, used to making human-centric games and will, from habit, not know to put more work into making aliens think alien and operate under different pretexts when it comes to war.
There's room for all sorts of alien reasoning, like a Klingon type race that values honesty and naked aggression and so has no need for CBs, and in fact, loathes races who use them, like Mankind, the most for simply making excuses to go to war and are despicable for it.
Another could be a Borg type race that looks on everything, and everyone, as a resource, and after laying claim to a word that is best suited for a certain type of race, gets a CB on it to go harvest victims to assimilate them and colonize the planet: They find a Earth-like world, place a claim, then discover Mankind and so are able to declare war to scour colonies for x amount of humans they need to colonize said planet and have enough to sustain a breeding population.