I have seen this many times in my >2000 hours (since everyone is posting this, lol).
I believe if you look around there was a revolt at some point and the land was sieged, however, the rebels left the land, encountered an army from someone and died. The land thus was sieged down by Greek Patriots or some such and in some cases rather than forming a nation that isn't around atm (Greece is a good example) it defected to you. I have had this happen 2 or 3 times now in 1.8 games and more times than that in the many other games I've played in previous versions. So it's not so much a mtth factor as a possibility when the conditions are just right - something that is fairly uncommon much of the time.
You would see this happen a good bit more, I think, if you did something like play a country wherein your enemy has many of your cores or lands with your culture, blob up, attack them, win the war, get it to where you occupy all their provinces and do _not_ do a peace deal. Rather, watch unrest and revolt risk for *their* lands, let it go up until revolts for your culture take place, as well as a few others, and then declare your peace and do take lands with your culture that ajoin the rebeling provinces. Once they have sieged down some provinces and they come on to your lands, destroy them.
Now, the tricky part. Your enemy has to not siege the occupied provinces. So optimally, in your peace, take provinces that split your enemy's main lands from the besieged provinces such that he cannot send troops to take them back.
If you are lucky and there isn't a country to spawn with the same culture as you (less likely in 1.8, more common in prior versions) or in some cases, even if there is, then it may well flip directly to you instead of forming a different nation.
I picked up a Circassian land as Georgia this way in one of the playthroughs i was doing to experiment with Georgia. It *should* normally have formed a new Circassia. Instead, it defected to me.
You'll thus see this quite frequently, albeit, in the *rare* circumstances that everything works out right for this. If there are still rebel armies around, I haven't seen it flip to me. So I am fairly certain the rebels being dead after the sieging is key.