How did this happen?

[Olaf The Great]

If Ankyra upgrades to a city before it is conquered, the Aedui will control it when it rebels.

[Arjos]

If Ankyra upgrades to a city before it is conquered, the Aedui will control it when it rebels.

If yours a question, than no it will be galatian eleutheroi...

[Ca Putt]

provided I understood him correctly he meant that if upgraded by the eleutheroi before it's conquered. as this is seldom the case it mostly rebels to the arverni.
that makes Aedui faction_owner and arverni faction_creator (well i did not check it in the code but that's what would appeal to me mostly)