How do YouTube related videos work?

Basically this video here http://www.youtube.com/watch?v=reLp8JzT60M has related videos that are extremely popular youtubers yet the tags and things are not really the same at all. Now ive been trying to work out how he managed to do this? I want to be able to have related videos like that but i have no idea how he managed this. If anyone has any idea of how he managed this, please help :) Thanks! My channel is http://www.youtube.com/user/RohaanSaleem