On the same token, while washing my car today (at the carwash) I saw a dude with his jeans all the way below his ass. Seriously, like his beltloop was around mid thigh area, just barely above the bottom of his boxers (which btw weren't stylish enough on their own to warrant us seeing them).
Why do they do it, and other behaviors you ask? Well the pants thing I know comes from jail, where they take your belt away, thus your pants droop. And being that rappers idolize the criminal lifestyle, they immitate it. As for the crotch grabbing, I hardly think they're trying to be like MJ...but are prolly using it for the same reason: shock factor, at least that's my guess. I can't seem to follow this line of reasoning very far!