The AI was stupid about surrender before, often forcing you to actually destroy all their planets even when they had close to zero income, no army, and little defenses on their remaining planets.  Now it seems the pendalum has swung the other way, and they fold as soon as they start to fall behind. 

I think this draws more to attention than just the fact that the AI surrenders on a hair-trigger.  First of all, the AI (on every difficulty, it's just higher difficulties get a resource cheat) mismanages its economy, usually resulting in a dramatically weaker position than a player empire by the 30-60 minute mark.  Apparently the normal AI falls so far behind it will surrender on this ground alone.

Personally, I feel that the AI should go into "desperation" mode when it detects itself being heavily outmatched, and start taking heavier risks like attacking enemy planets even though it may not have a military advantage.  It should surrender when its fleet is mostly destroyed and it lacks the economy to replace those losses.  In other words, fight until it no longer has the resources to do so.

I as well have had even the hardest set of AIs serender at even the slightest hint that it was losing.  I was just playing a small map with only one very hard AI, I took about a third of the planets then started to bunker and prepare for a major assult, but before i could, it sent a huge fleet to one of my better defended planets (also the assult fleet I was still building up was there), but the fleet it sent was only made up of light and hevey line units, no support units at all, and only 2 or 3 cap ships.  My defence and the fleet at that planet was well rounded and chewed the AIs fleet up with only light losses, about a 3rd of the AIs fleet excaped.  but as soon as the battle was over, it serenderd.  It had more planets than i had, and still a decent sized fleet left.  after I ended the game, i checked out the the resorces it had.  More than me.  The AI had the upper hand, and surrenderd to me.

Some ideas i had to help with this:

Adjust the AI to only serrender when is unable to combat, i.e:  low resorce income and very little left in the bank.  no fleet.

when only few planets left  and with very little defence, but if it had good income with good defence, keep fighting. 

let players and AI to lower the amount of fleet points it has (thus reduseing the fleet upkeep tax) or adujst the up keep tax to how many ships are built, not how many can be built.   The incresed amout of resourse income will help rebuilding after a hevey loss.

during one on one games, or the with the last serviving AI, no surrendering what so ever.  To win you would have to reduce the AIs planet count to 0