It could happen. But, in most of the cases it is because of the flight has been oversold at least 4 days before departure. The expected no-show did not happen (for certain route at certain period the airline expect certain number of sold Y seats not to be taken, often the prediction can be wrong or just close). The second possibility is reconfirmed groups seats are underestimated. When a group reserved spaces before final confirmation of the number of passengers, the airline already sold large number of Y seats to FITs at a higher price. When the group final pax lists come higher than the estimated airline have a situation of lower yield as well as potential oversold. If J is terribly empty the airline normally will try to sell more high Y fare to mark up the yield without turning away the group passengers. Last minute Y fare in international travel nowadays are rare in between.