I have fought other races down to the last planet, keep them at zero ships and they didn't surrender, but when the npc races seem to fight each other they surrender alot, and alot of times its for no reason I can see.
What makes an NPC decide to surrender can be pretty sketchy, one time they will go home crying to their momma after only a few shots have been fired, and sometimes they will stay until the very bitter end. This is not really player-specific though, but more a matter of chance and moment in the game. Some races may fail to even build up a military, and then realise that victory is hopeless and surrender instead. In my experience, the last few races in the game will not surrender at any cost, to anyone.
As for my op, the race with one planet was left over from my last war with them and keep them as a place to suck the last science out of them via starting a war then they ask for a cf after a shot time and I usually con a science out of them. But the race that was now in control of about 1/2 the galaxy surrender to the single planet race after I only took 2 planets.
The why of surrendering can be pretty random. A situation that was reason to surrender one time, may not be sufficient to trigger surrender the next. The AI is programmed to not be completely predictable, or you'd end up with every race always surrendering under always the same conditions.Ask yourself this question: the race that controlled half the galaxy, did it really stand a chance at beating you? You took three of its planets and it seems to me that your rampage would pretty much go unchecked. Under those conditions, is it wrong it assumes the worst and instead surrenders its population to the AI that often is at war with you, but no longer loses a single planet during those wars?
The npc races just seem to do stupid things to and with each other. Most of time I go to war the enemy surrenders to anyone but me, yet when a npc race goes to war the ones they war seem to all ways surrender to them.
This I cannot subscribe to. The AI doesn't seem to have a player-based avoidance of surrendering to. In my experience, they usually either surrender to the one they are at war with, the weakest other AI/player, the strongest AI/player or... well, anyone really. I have had plenty of moments where I was "brought back" by inheriting a failing empire that doubled mine in size, from someone I was not at war with. In the late game, the AI really doesn't like to surrender anymore, especially if there are only a few competitors left, and even more so when they are the last AI standing.
I like the game in general, just wish star bases didn't need so many construction units to build them up. You would think you could have 2 options speed build with a fleet of construction ships or a slower self build option similar to building a ship on a planet. Overall a great game.
Quoted for truth. Starbase construction can be a huge pain in the behind and require vast amount of constructors. However, as they are also potentially very valuable, this is not always unjustified. In the early game and the early midgame, starbase defenses can really decimate the hostile fleets that will keep trying to destroy it. The resource bonusses that can be extracted from the different resources can also be very potent and should not be too easy to access. Economic/military/influence starbases really consume a very great amount of constructors, making you doubt if they are really worth it. Also, in the late midgame and the late game, it would've been nice if starbase weaponry had been streamlined so you no longer had to plough through all the small boosts in weapons to get it semi-defensible, but alas... 
