According to 'The Basque History of the World'

Suleiman, a Muslim who was feuding with the emir of Cordoba...wishing to take the Ebro away from the emir's control, had crossed the Pyrenees to offer Charlemagne a list of cities...that he had arranged to have fall to the Franks without a fight

The author then goes on to explain that the real incident behind the Song of Roland is actually a Basque ambush, in revenge for the Franks attacking Pamplona, rather than a Moorish one.