This fashion trend originated many years ago in the poorer neighborhoods, where most could not afford cars, or public transit to get to work, so they rode their bicycles instead. The pant leg on the right side was rolled up to keep it clean and prevent it from touching the chain of the bike or getting suck between the chain and the front sprocket.
One of many current fashion trends derived from practices of lower-class memebers of society used several decades ago.
guy 2: Oh, did i forget to roll my pant leg back down again?
guy 1: Yeah, you got ghetto leg