I was informed it was due to the sexual nature of the prision enviornment. Men would half show thier a$$es to the other inmates for...well, let's just say it wasn't for tea time thats for sure!
It has since spilled over into the streets and has become a fasion statement. ( For God knows why...)
Ironically, where I'm at, there is no sign of 'low riding' what so ever. Moslty rich kids whos parents can afford A&F, AE, Holly, and the likes. ( Hell, I can barely afford wally-world! But, that is another story. )