Yes, sounds like you already know. For instance, in the case of the Paris Hilton and Kim Kardashian sex tapes, those were leaked/sold by one of the two people in each video. In the case of "The Fappening" leaks, multiple people had found an exploit for Apple accounts that let them keep grabbing images from celeb's iCloud uploads, and some guy who was paying these people to grab them for him just decided to dump all of his folders of images. That was on 4Chan if I remember correctly.
This is the same for non-celeb or e-celeb leaks by the way. "Revenge porn" is a thing, so once again someone in the video would be leaking it themselves in that case. And getting hacked works the same for everybody's account on the planet, regardless of fame.