It's was is happening when you start to get the upper hand on another European nation. Once you succeed in beating them, you are better completely absorbing them. Ok fine for now. Then, once unrest is gone, all of the sudden, you get dozens upon dozens of expert citizens from their former cities, complete with several very developed cities. It changes drastically your power curve and makes resisting the Independence war almost trivial, because success appeals success and once the first European is beaten, the second is even easier... I'm playing with house rules about that, but it feels almost as a cheat to me to beat an European, then get rewarded with a productive population within 10 turns that will double or triple my power in one go.