Most externally it's part of the Latissimus Dorsi.
What's important to remember is that the Lats lay over the bottom portion of the scapula, which means that the lower parts of the teres major and minor, infraspinatus and even Rhomboid are covered by this top-most part of the lat as well.
That is a lot of moving and flexing parts which, under extreme stress, can really extrude this part of the back. That + protein shakes and heavy lifting make for a high definition band-like look here.
Notice how in this link:
that band-like top portion of the lats disappear. The arms are at rest and so they are inactive.