@Mor - I've heard that theory before and I'm sure that if you trace back recent viral videos you will find that something similar happened.
But when people look for theories about why something becomes popular, I think they overlook the idea that sometimes something is just really good. I only share something if I think it's worth sharing. I think if a video is of a certain caliber (and I don't mean in terms of quality, just in terms of entertainment value) then it will eventually become a viral video, even if it doesn't happen right away.
Promotion makes it happen faster, but there are plenty of boring videos that have been promoted heavily and never caught on, because no one cared.