Kevin Allocca, YouTube's trends manager, shares the 3 reasons why a video goes viral. In this video he takes the example of three videos and goes on to explain how, tastemakers, communities of participation and unexpectedness are the reasons on why the videos went viral online.
Another interesting thing to note in his talk is the role played by twitter. Some of these videos were quite old by the time they went viral and how a tweet from a tastemaker resulted in the video going viral. Do watch this interesting talk.