This explains exactly how something can go viral. Not the video itself, the event it’s recording. Chance is a big thing. If an ad agency says they do viral videos and campaigns then they’re talking bollox. You can no more guarantee a video will go viral as you can getting your site first on Google in a competitive search area. You can facilitate making it easier to happen alright.
Watch this video as a lone drunk/on something else dancer is joined by two more people as more and more people just watch them. Suddenly within a few seconds there are dozens dancing with them. The surge of the crowd is almost like a riot.