Multiple tattoos and body piercings were once frowned upon, but now getting a tat is virtually a ritual for most teenagers as they approach the age of 18. The same can be said for sagging pants and visible underwear (originally a prison message for enticing homosexual sex partners); today, many misguided males have reversed its previous meaning, instead believing their droopy drawers clothe a macho and not a gay man.
Sometimes things are looked down on because they are just different. In some cases, there are others who do not look down of these same people, but admire them instead. In this case, they copy the deviant behavior because they like the person or because they like the behavior.
I agree with post 2 that what seems to be so radical for a time becomes the norm later simply because time adds to its acceptance and the youth who once were considered rebels or deviants become the new accepted culture. Music, dance and even language on tv are good examples of this. As the accepted language of today's tv shows illustrate, we no longer worry about George Carlin's comedy routine entitled The Seven Words You Can't Say on TV. I've heard them all, and no one seems to care at all as they are now accepted as mainstream tv.
I think that it is simply because the people who have been doing the deviant thing grow up. They become the new setters of cultural norms. This can be seen, for example, in how rock music started out as a deviant thing and then became completely mainstream.