Just a guess, were the bags (assuming they are plastic here) breaking down from the sun? If so, perhaps particles of plastic from the bags went into the hopper with the coal. As it warmed getting closer to the fire it melted leaving a gooey mess that hardened like rock once the fire went out? When chipping it out, did it seem consistent with what you would expect from a plastic?
Anthracite is the Chuck Norris of coals, I doubt the sun could hurt it.