The Korx just surrendered to me.

Never can tell who an AI will surrender to.
You may have been on better terms with them, than the others.
Perhaps they liked your economy or alignment or something.

Just be glad for the extra planets, and make good use of them.

You don't need to be at war to be surrendered to.
Basically they submit themselves to you in a final attempt to keep their planets out of the hands of the enemy. Sometimes though, they will surrender to the attacking party and just give up.

Yeah, it can sometimes be a boon or a bane. Had one game where one of the races surrendered like 10 planets too me. Not a one of them were economically balanced. Almost totally ruined my economy.

Other times it is like mana from heaven. High research/econ planets just from the ether.

Also, the event isn't always repeatable. I had one game where a race surrendered to one of my enemies. Game CTD (for unrelated reasons) and I had to reload. Next time, the surrendered to ME. Happy day.

The AI almost never surrenders to the civ that it is at war with. Odd behavior, but this is the way the game is written, so there you go.

pitty you cant free a civilization; just like in star trek botf.