In my opinion, I have to say that Lancelot bedding with Queen Guinevere, King Arthur's wife, was the start of the cracks that Camelot eventually crumbled from. However, even in the tales, with the squeamish writing of the medieval writers, when they talked about their "bedding", they say that love was referred to differently in those days. Yes...
However, in my understanding of the stories, it seems that everyone was in on Lancelot's nightly (knightly?) escapades. The whole Round Table was in on the the supposed "secret!" It's like some sort of bad episode of Desperate Housewives.
In my opinion, it is the knights who reveal the affair to Arthur who cause Camelot to fall. Everyone else was quite happy to continue on pretending there was nothing going on between Lancelot and Guinevere. Arthur apparently didn't have a clue what was going on in his wife's bed chamber. The downfall of Camelot could simply be put down to jealousy among the knights. Lancelot was Arthur's right-hand man and confidant, and some of the other knights were envious of his position.
Arthur's tyrannical reign in later years, and especially after he found about about Guinevere's affair, certainly added to his downfall, along with Camelot. He orders Guinevere, his WIFE, to be burned at the stake for her affair. This causes him a lot of bad publicity and the defection of some knights. Lancelot rides in on a horse and saves Guinevere, which earns him some extra points with the other knights.
Arthur's reign had ended by that point. He further compounded his situation by refusing the Pope and by fighting his own son to the death. Arthur's story ended like a Shakespearean tragedy, and Camelot is forever lost.