Muscle groups are covered by thin connective tissue sheaths. You can see these as Seams between muscles. This connective tissue, made of collagen, is covered in fat on the portions next to the skin of the animal. We call this the Fat Cap. As you cook the meat the connective tissue cover, with a portion of the Bark, shrinks and exposes the muscle below. This is what you are seeing. Totally normal...JJ
Thank you all for the comments and advice. It came out delicious.