I know some rule variants don't allow the game to end unless at least one player is above 30,000 points, but it's hard to imagine you went thirteen hands with everyone being relatively even.

Maybe just ended in draw... In some conditions a draw make the round start again, until someone wins. I found this:

Renchan (連荘) is a situation in which a player successively plays the dealer, and is often only caused by dealer's win or tenpai; therefore, draws are not always renchan. On the other hand, the number of honba always increases when a draw or a dealer's win occurs.