I would believe it is a reaction with the oils that have been used in the bore. Especially if it were an oil based on real pertroleum or containg Teflon.

Even though the surface of the bore may have been cleaned with alchohol the pores of the metal still contain contaminates with can react with the heat of a primer as well as the heat of the load.

I wonder if he were to clean the bore with boiling water, which will case the pores in the metal to purge their contents then try the same experiment, what the results might be?

If the fouling were soft, I would believe that an additional problem will occur when shooting just primers, because primers are not clean.. and the debris build up will naturally occur around the nose of the breech plug where there is a ledge for the primer pressure to build back on to.