Actually, the first answer is somewhat right. I did research on why men sag,
however, and found the origins indeed to be in prison - but for a whole
different reason.
In prison, men tended/tend to have sexual relations with each other since
female companionship is lacking. One way men "showed interest" in relations
was to sag their pants, letting other men know they were willing. So,
basically, its root stems from a request for gay male sex in prison. Every
time I see a man sagging, I burst out