I'm way ahead of every AI on the new world (some aren't even in medieval era yet and I took norsemen before dutch).

I was about to attack this independant people city:

As you can see, AI player #3's superpowers make him close to assimilate the town but I know that and that's not the issue I want to talk about...

On the next turn, out of nowhere, AI player #4 assimilated the city:

AI player #4 is Hittites with 1 territory, vassalized by AI player #2 (Mongols).

He can't be there he doesn't have the technology nor army and had no claim on the city.

Did AI 4 (Orange player) conquer the city by moving an army in that fog of war ? I am asking because the population of that Neutral city also jumped from 2 to 12 in one turn, so something definitely happened, not only being taken over by an AI. 

No they still have their territory, they are vassals of AI player #2 and now I have a grudge with AI player #2 for taking my independant people, but it's the same issue, Mongols can't be there either.

I see two possibilities:

- Military attack from the left, out of your visibility

- Or #3 green toaster assimilated the city, received a grievance, #4 asked the territory because "Dude, I discovered this natural wonder before you!", and #3 gave the territory to player #4

Edit: for the pop jump from 2 to 12, that is odd, maybe an assimilation of the neutral armies, 2x5 pop?

@globule
We need confirmation what armies did the Independent city have on turn 182. That is still a pretty big jump in population.

@SpectrumWarrior
Did Orange have any armies close by ? I edited my message as this was the most probable reason how the Orange player managed to get that territory. They do not need boats to get there, they can hire mercenaries.

So they hired 2x5 mercenaries, took the city and disbanded them into people. Clever.

But for that, wouldn't mercenaries have to come from the city? You can't disband mercenaries at will and they had no patronage here.

Annexing is different from Assimilation. Annexation is only available to Expansionist cultures and the Hittites are considered a Militarist one.
Something is off with your game, the Hittites should not be able to annex anything. It also only works on Outpost and Administrative centres, not on a city's main plaza.

P.S.

Can you please provide a save file from turn 182 ?

Hollandais Tour 182.ctr

You can pass the turns of every of my armies, they'll still get the city.

You'll see there's a grievance with AI player #2 (Mongols) for his annexation of my independant people.

If you declare war with Mongols, then you'll get a grievance against Hittites for their annexation of the city...

edit: frenglish: annexion / annexation

I am curious, will try to test this save! Unless it is a grievance demand, that is a little odd. And same for the pop