If you're very sure you haven't just accidentally set an opponent to Team during game setup, it sounds like you might have a borked installation that somehow mingles a campaign scenario with your sandbox settings, but I've never heard of something like that and I read a good bit around here. It definitely doesn't sound right.

Under the window that shows which opponents you have set (last page of game set up), you can select your starting relations with individual civs. If you started every game allied with a particular race, I'd say that's exactly what got changed.

Since one of the settings here is "random" it's possible you're just rolling one teammate by random each game, but the odds of that are absurdly high. I'm not even sure team is possible to get out of a random roll.