Look at the top case; you will see that the scrap is being caused during the ejection pull since the end of scrap is the area with the build up still left, and the sharp edge, the start point.
As for what is causing it, the lug tip at the fore most ejection port side of the receiver extension has a sharp end bur, and as the case is being withdrawn from the chamber (chamber no longer supporting the case*), the case is being kicked to this lug and scrapped down it on the ejection.
*When the case is withdraw enough to leave the just the neck section in the camber, the lug bur makes contact at the beginning of the shoulder, then as the neck clears the chamber, the case is allowed to even more misalign with the bore line (90* off the bolt face), to allow the lug to dance off the spent case a third time at the neck (think of it as chatter when machining).