I've never understood how it became 'cool' to show your butt.
couple things... think about being in an alley and you and a friend are out looking for trouble... you see a lone guy with his pants low - obviously you cannot see a knife or gun in his underwear band, so you think maybe he is an easy target... but no, you see that his pants are around his ankles, which makes it impossible for him to run... which means he will stand his ground when the time comes, cause he has PLANNED to not be able to move right, or quickly.
The pants like that screams 'I am not going to budge when the time comes, and I don't need to worry about you, or carrying (unless you do see it in their underwear band) and I will mess you up if you try anything'. In other words... it's an attitude and mind-warefare like a bird puffing up it's feathers for territory, or a dog's fur standing up.
That and the prison pants hanging low due to fitting, and moved on to the hand in the pocket thing, meaning if you mess with a man's woman.. he will come looking for you.