It's because prop lightmapping maps a single lightmap to the UV, which in this case is just the regular stone texture. Unless you're super careful, props generated from brushwork contain multiple instances of their texture, so VRAD just goes with whichever one comes first in the file and then repeats it everywhere else. Probably it started with a face that was clipping into the wall; either way you're not going to get a good lightmap out of this arrangement.