There seems to be no logic when to the surrendering, just random chances between all the AI players (and a very slim one to you)

Is it possible the Yor had a lot of influence in Arcean terriroty?

It does seem random as I have reloaded a savegame just to see who they might surrender to if they had the choice again.

Anyway, I don't think races should ever surrender to a race that they are not at war with. This can also prevent accidental surrendering - however you should still be able to give away a planet for free (not the same as surrendering).

It is within the realm of possibility but very doubtful, the Arcean worlds were spread out because the Altairians had surrendered to them earlier in the game. It appeared that I had most influence as Terrans, the Yor were listed by the computer as being the most powerful at the time. I wonder if that could increase the chances? At any rate, the Arceans were not at war with anyone at the time (there were no wars going on actually). What is the point of surrendering if your not at war with anyone at the time?

i would like alittle more logic on this myself, semms really odd as it is now

The AI doesn't seem to know the difference between losing ships in enemy territory and having an invasion fleet parked right next to their planets. Plus the guy (Earth), surrendered to the Yor, the "we want to kill biological life"-guys...kinda odd. It is kinda weird that conquered planets or ones surrended to the enemy don't have a morale penalty. I had to use conventional warfare because every planet had a 100% approval rating (and I don't like damaging planet quality or infastructor too much). The Carinoids (however that is spelled) were right next to the humans, and they whined every 10 turns or so about how they were about to be defeated. Even though there was no chance that was going to happen. I wish you can tell the AI "they'll attack your planets over my dead body" or something else like that. It seems they are only happy if you give them something.

This is probably tied for the single worst "feature" of the this otherwise great game, the other of course being that the AI doesn't know how to attack first or build ships with engines

How's this for a questionable AI decision... I think it must be a bug:

I was playing a game, all enemies set to Intelligent, large galaxy, 7 opponents. Playing as the Yor, I had conquered the galactic-north bordering Arceans, and two Minor races, and had taken a few planets from the Terrans, my western bordering neighbors. War had been declared on me by the Thorian-Altarian alliance, so I graciously offered peace to my Terran neighbors while I dealt with the new, far more significant threat.

30 or so weeks later, two more Terran worlds have succumbed to my influence, leaving them with only a handful of worlds left. I have talked the Thorian-Altarian alliance into peace, and am rebuilding a next-gen fleet for a major offensive against both of them. The Terrans have almost no military might, but they are on great terms with the "Alliance", so I decide to wait to conquer them. Lo and behold, out of nowhere the Terrans surprise attack me with their pitiful fleet. At the end of the turn, they surrender. What the heck is that? I actually laughed out loud. It just made no sense at all. Oh well, maybe it was a bug, but that whole surrendering thing really needs to get worked out.

The funniest thing: They surrendered to me. WHY?!?!?!

I find it unrealistic that other races surrender when they're allied, since they would not have to stand alone in a conflict...? I hope it would be possible to make less likely to surrender if you're allied?

TIP

If I'm Civ A, and I'm rapidly conquering Civ B, and Civ B decides to surrender to Civ C out of spite, then Civ C needs to decide whether or not it is scared of my (Civ A) military before it decides to accept the new worlds. In my last game, once Civ B surrendered to Civ C, it was easier for me to just keep rolling into Civ C than it was to put the brakes on the war machine.

I think the surrendering is based on the culture similarities and if the people like each other. They also seem to surrender to the safest civ, eg the most peacefull or most powerfull. Or sometimes they surrender to your worst enemy just to spite you.

This atleast, is what seems to happen in my games!

I love that feature. I think it's hilarious! You know they are doing it out of spite, and I think it's awesome! In every good 4x game there is always something that upsets the balance. In Civ4 it's when a critical resource dissapears and you have to declare war on somoene you didn't want to! In this game it's aliens surrendering to a different CIV! I love it!