Why do so many girls/women walk around with sweaters wrapped around covering their butts?
Are they prudes and don't want guys to look? Are they self-consciously thinking their butts are either too big or too small or flat? If huge, that isn't fooling anyone. The "reason" is that it is convenient in case they get cold, doesn't wash. Most have humongous bags or packs to carry them in or they could drape over their shoulders.
I think it depends on the person. Some might do this because they are self-conscious, others may do it as a fashion statement. Wearing a jumper around the waist is seen as being more fashionable (by young women at least) than wearing it over the shoulders. And some of us women who do carry huge bags already cram them with stuff so there's no room for a jumper
If the weather has heated up (or I've been doing sports or have gone for an early morning jog) it's too hot to tie the jacket over the sholders and wear it down my back, so that's when I'd tie it around the hips. It's not a fashion statement for me, just more convenient when you don't want to wear it or carry it.
I know that when I was little we all did that because it put the sweater out of the way, left your hands free to do what ever. Since elementary school I have not worn a sweatshirt this way, it was also a fashion thing. You'll notice when you watch shows like "Full House" from the '90s all the kids were dressed this way.