What cause the death of Roland in the epic song of Roland?

Ganelon is infuriated, feeling that Roland has assigned him for this perilous mission trying to be free of him for good. Ganelon has for quite some time been desirous of Roland, and on his strategic mission, he plots with the agnostics, revealing to them that they could snare Charlemagne's rearguard as Charlemagne leaves Spain. Roland will without a doubt lead the rearguard, and Ganelon guarantees that with Roland dead Charlemagne will lose the will to battle.