Its happened many times with various civs. Pretty sure its a bug. It only happens with chariots. The civ will spawn at war with me and will have barb-flipped chariots. I have started probably close to 20 DoC games in the last week and a half, all with early Mediteranean civs, and its happened to me about 4 or 5 times.
I don't get a choice to take over the civ, or any first contact, just a chariot on my borders and then the civ appears with the list of civs and I'm already at war.
On another note, have played for Greece new UHV a few times now. It is very hard. My latest attempt I am parked outside Parsa with an army on turn 119 after taking control of all other areas, but can't take the city.
You need to be able to take Babylon and Egypt with your initial hoplites which is tough if they have teched to archery. Otherwise its really difficult to build an army back in greece as well as the wonders. Definitely doable but very tough, and you really need luck on your side. If both Babylon and Egypt have archery when you spawn then you can't defeat them without city-raider hoplite reinforcements. I then use my initial units to prevent them from getting to their marble while trying to build reinforcements and get to the Oracle first. This also requires you settle epidamnos for the copper. It hasn't worked out for me. I either get bogged down in Babylon until Persia spawns but get the Oracle, or I manage to get an army to take Babylon but lose the Oracle. Not sure who's building it (India?), as I have units parked on Egypt's Marble. Getting a strong point in the Middle East before Persia spawns is key, even better if done before Phoenicia.
On the other hand there doesn't seem to be any rush to get the lighthouse or the parthenon. The Oracle seems to be the only wonder other civs rush for from Greece's required wonders.