S'cuse me Lis, but I'm one of those neanderthals who wears their jeans 15 cm below their boxers - my reason is that I have no bum (my uncomfortable saddle has ridded me of that physical feature)... (but I never wear C K) (and I never show bum crack - don't like the breeze that cycling causes down there...)
If you want an explanation of how it started: - the most generally accepted reason is that in LA, all the little kids in gangs would wear their bigger brother's clothes as a sign of respect... basically, the kid in the baggiest clothes had the biggest older brother, so the other adorable little youths wouldn't pick on him... (or so the story goes)