I have noticed the same thing happening to me since installing the Entrenchment expansion.  Usually it occurs once I attack the HW, but I had barely started and hadn't even wiped out the population.  It is rather confusing... perhaps to mimic sore losers that quit as soon as they realize they cannot win...

the AI in Entrenchment surrenders under a wider variety of conditions than it used to. i don't know the full list but this is what i've noticed. the AI becomes more likely to surrender when:

1) you nuked their homeworld and have a much bigger fleet than they do.

2) you have built Super Weapons (Novalith, Kostura, Deliverance) and they have nothing comparable. 

3) you have multiple high level capital ships and they have no high level capital ships.

4) in a team game if you have defeated members of their team so that they are facing a 2on1 or worse situation.

5) you have built a Starbase in their homeworld grav well (or maybe just one of their highly developed grav wells anywhere). 

6) they have just lost multiple colonies in a short period of time (fairly likey if they are being attacked by more than one player on more than one front). 

their may be more conditions than those, but i'm pretty sure each of those conditions significantly increases the chance of a surrender occuring. they seem to stack with each other as well. in other words the AI has become more proficient at identifying situations where their loss is basically inevitable. 

you will play fewer complete games with the AI than you used to. none of those conditions are automatic triggers, they just increase the chance of a surrender. some AI's will just be stubborn though and fight it out. i think Aggressor AI's tend to surrender the most often and Defender AI's are the least likely. 

I can understand some players not wanting to drag out a game where victory is inevitable. However, I would much rather prefer getting a message akin to "Player X wants to surrender to you, do you accept?" Unless everyone accepts, the game goes on and the surrender is void. This would make those on both sides of the fence happy. Any idea how to mod something like this?

I was in the home stretch of a 3 system 7 player match with just me and an Advent AI left. The Advent still had control over half the star system we were fighting in, had a fleet rougly equivalent to mine (though I had more capships, they had many more frigates and cruisers along with a hefty number of drone carriers). I was prepping one of my two fleets to jump at a desert world with an artifact when a lone drone carrier hopped into the grav well, just in range of my 8/8 starbase.

The moment the carrier went down, the AI surrendered.

I scouted the map afterwards and saw that their homeworld had just been hit by a Novalith shell (I leave my superweapons on autofire) but was otherwise still intact.

Perhaps an option in the startup to disable surrendering? Fighting against an opponent with no fear of retaliation is NOT FUN.

Please, just give us the option to disable/activate AI Surrendering. It should be possible to change this option at everypoint of the game or at least on loading a game.