Why did Christoph Willibald Gluck destroy his sketches for the opera Roland?

Christoph Willibald Gluck destroyed his sketches for the opera Roland upon learning that his rival, the Italian composer Niccolò Piccinni, had been commissioned to set the same text for the Paris stage.1

This decision occurred during the height of the rivalry between the supporters of Gluck and the supporters of Piccinni, a period often characterized by intense factionalism in the Parisian musical scene. While the two composers were frequently pitted against one another by their respective partisans, neither Gluck nor Piccinni actively participated in the dispute themselves.21