Let me direct you to some detail discussions surrounding Leader Appearances
Maximising Leader Appearances
After long discussions, theories and even myself asking MikeB, it all come down to random chance.
Or so they say.
In my last two games, Leader Tournament 3 Game 1 and 2, I've had different experiences asfar as leader appearances. In the first, playing as the Indians, I was playing relatively peaceful, going for cultural victory, and I had numerous early leaders. In the second, playing as the Aztecs and going for fullout early war, I've had 3 and it's 500AD!
Just look at this:
(sorry for the badly cut pic, but it is bit of a spoiler)
You will notice that I've had rushed the Pentagon, Forbidden Palace and the Heroic Epic by 10AD. That includes having 3 armies. The reason for creating so many early armies: THERE WERE NO WONDERS AVAILABLE TO RUSH!
In my Aztec game there were!
(Just a new theory I'm forming
)