This is the first time it has happened with 15 games. It happened a couple times back when there were fewer teams and games per day. So this is really the first time since 1998, when the last of the current 30 teams was created.
Given 30 teams, 15 games, the difference in each season in the win/loss records of those teams, and the innumerable possibilites involved in baseball, it's actually astonishing that it only took 17 years for this to happen. Mathematically, it was far more likely that baseball could have continued as-is for another century before this occurred.