We all know that the Geass is a great and amazing power even if they have such limitations but if the user is lack of willpower he/she may comsumed by it and lose control on it like Mao, his lack of willpower turns his Geass to Geass Runaway..
but, in case of Lelouch that have a great will to down the Britain Empire and to make peace not only for Nunnaly but for the whole world..he also lose his control on his geass..but why? And how? The answer is never reveal in anime/manga.
So in my own opinion..it's because of Lelouch's mental instability in that period of time. Or because of excessive use and reached it's limit. Maybe both?
What's your opinion?