It was always there. It was almost even systematic once you started to play at the Information era without any input from the players: sooner or later, most of the AI will bankrupt themselves at one point. Too much military at the start, not prone to build Commercial Hub / Harbor, nor slots the policy cards that could helps them to mitigate the problem, and no other civilizations encountered yet to send them Trader for some cashes. Fortunately, they will bounce back.
If you start the game at an earlier era, it needs you to aim at bankrupt the AI on purpose to even witness this. It is very unlikely but not implausible to bankrupt an AI unknowingly by selling them your luxury resources. So I don't know why you are witnessing this suddenly. Are you playing game mode like Monopolies and Corporations? Or Zombie Defense? Maybe those are hurting the AI much more by not improving Luxury Resources and pumping more military making them more prone to bankrupt themselves? Or did you use a mod that makes trading with AI more confortable (so you just trade more with them at the best price, who knows!).
Or are the AI more aggressive with Spies and Fund Siphoning?
I've never not done an Ancient Era start, to my recollection. I've never used a mod. I generally do use M&C, so maybe that, although it's not every game it happens (it's almost always on) but it when it does happen, it's often multiple AI. I only ever did the Zombie mode once and didn't like it so I've never played it again.
Current game I'm in a war with Gilgamesh (that he started). He basically went all in on invading me and failed to take down my walls and he lost all his units. I've started attacking him now and it seems his entire strategy is to send envoys to nearby city states and levy their troops. Of course I have held back on using my envoys until after he does, so the end result is (I'm fairly sure) he is just running projects to get gold, spending it on levying, then losing the levy the next turn.
AI levies a lot more than humans, so it could just be levy expiring and not being able to be renewed.
This reminds me of a change in AI behaviour. It used to be relatively rare for me to fight levied units, whereas now it seems like every war involves them.
Now, thus might not be down to the AI itself changing. With the release of the Huge Earth TSL map, I've been playing huge a lot more *I always used to play Standard). One thing I've noticed is that CSs , especially in Europe, are significantly denser. Despite Europe doubling in size (to my estimation, possibly it trebles), there are a lot more CSs - usually there aren't any on Standard, but I my Huge map fromnmy last.game had Granads, Brussels and Venice. Other areas aren't quite so drastic, but I get the feel that there are more CSs for the same number of hexes, or at least settable hexes, than in Standard. As a result, a civ is more likely to be sharing a border with a CS.
This might make them more likely to levy units when attacked. Levying units is pretty expensive (several hundred gold for thirty turns, generally), which would put a strain on their economy. I mean, I looked at their gold levels yesterday at the end of my last game, and it looked pretty low throughout. A sudden cost of a few hundred could easily wreck their economy.
So, theory: They're being attacked and because they are close to a CS, they levy their troops because if it saves them, it is worth losing that gold. However, this puts them I the red and, as per the explanation earlier, this causes them to disband their own armies because the algorithm tells them to keep disbanding until their account is positive, which never happens. All they have left are the levied troops. 29 turns later, the levy runs out and voilà, no military strength left. Their economy has recovered though, so they start building units again, and they get their old military strength back. Which is what we observe.
How we can test that, I don't know. We wouldn't see the sudden drop to zero until after the levied troops revert. Perhaps when we notice the drop, we check their empire for CS units, indicating that they had recently returned from being levied and are now back under the control of the CS? The post game graphs don't show military strength if I remember correctly.
But that would explain the mechanic, and why it's only started recently, at least for me.
