At one turn my bases located in another CIVs territory suddenly switch to that CIVs side.

First I thought this is a bug, then assumed it might be a missing notification which normally informs players on deserting assets but in larger amount.

Reading through the individual ideology bonuses found that one of them fits this case exactly: "Benevolent\Prominence\Enticing"

As there was no notification whatsoever what happened, the triggering of the above ideology bonus by the AI is just a guess - please insert notification if so