Similar thing happened to me in my first game: 'accidentally' unlocked about 3 Civs by improving various resources or settling in the 'right' places. Thought the progressions must be broken because I had so many choices!
Bu they only apply in that specific game.
Good news is that every Civ seems to have an in-game Unlock, so you always have a (sometimes faint) chance of unlocking them.
Your unlock sounds like Chola, which requires 3 City Centers next to coastal tiles - lakes not counting. Improving 3 each of Hides, Iron, Silk, Pearls, Wine, Tea, Salt, Jade, or Camels will also get you in-game Unlocks for various Civs, or settling next to Lakes, Mountains, Navigable Rivers or on Tundra.