he probably wasn't intending to change fully there... maybe just taking the top part off and tying it around the waist while he wear a t shirt instead, or he could've have had a heart monitor on or something and wanted to get it off..
but this is invasion of privacy.. at least it was not a zoomed in version like at the nhk/nationals(?) (which one I forgot but it was in Japan lol) and that had a close up view of his back