This would have been a perfect theme for No Pants Day ("it says 'no shirt, no shoes', but it doesn't say 'no pants'..."), but alas, that was months ago. Oh well, you don't need a holiday to celebrate freedom from pants!
No pants? No problem!
It's also a humorous riff on the popular-among-nudists fashion faux pas known as "the shirtcock". Its prevalence is probably due to its convenience and practicality - if you're cold, it's more effective to wrap something around your shoulders than your waist, plus it aids in protection against sunburn. Additionally, nudism offers a unique opportunity to leave your "nether" regions unencumbered and open to the air, so that's probably not going to be the first thing you want to cover up (else you could have just stayed at home).
Of course, "the shirtcock" is a nearly universally unflattering combination (though nudists, generally speaking, are largely unconcerned with appearance). That's why I try to avoid it as much as possible. Contrasted with a t-shirt, I think this hoodie actually looks pretty cute all by itself, and even better when I can leave it unzipped. At that point, I'm still practically naked, but with a little bit of protection from the elements, and the slightest feeling of being dressed (even as I am still totally exposed). In fact, that's precisely what inspired this video/photoshoot - the sense of getting dressed, yet still being naked!