It has happened to me a few times (mostly when I'm playing an Evil race) that a race will attack me because they think it is lucrative to do so. In this case it was the Thalans. They declared war on me, I invaded their single planet wiping them out.
A bit later the Drath and Altarian alliance simultaneously declared war on me because they felt my civilization wasn't worthy or surviving. I managed to defeat both races but this left my race in debt. Midway through the war, I changed my alignment to "Neutral" this however had no effect on how agressive the other races behaved toward's the Yor.
Now the Korath say that because of what I did to their Thalans friends, they felt their days were numbered and declare war on me. Now I didn't start that war with the Thalans why is the AI reacting this way is beyond me. The Thalan's declaration of war is what caused them to vanish so it doesn't make sense for the Korath to copy that stragegy.
After I defeat the Korath, I ally with the Korx since it is the only race with a "close" status to mine. The Torians declare war on them and to respect my treaty I have to declare war on the Torians.
The Arceans then declare that my wars of agression won't be tolerated. How am I fighting a war of agression when I'm defending an ally that is victim of an attack ? Why didn't they react when the Thalan, Drath & Altarians waged a war of agression against my race ? This was the only war I declared all others were declared upon me. Besides the Arceans periodically attacked minor races without repercussions from the UP.
By the time the Arceans declared war on me I was paying 1344 bc / week and my net revenu was -600 bc. Their military had grown quite strong.
So I used my intrinsic abilities as a Yor to pull a Kobayashi Maru and change my amount of BC's in the my save game. My debt became a large positive amount. This allowed me to reimburse the debts and take care of the Torians and Arceans.
I just find it odd that the person being attacked is considered the aggressor.
One strange thing that happened was that the minor races gave me some of their ships to fight back. Usually they aren't available in the trade window. I know that the AI races can trade for minor races ships because I periodically see those ships in their inventories.