Metadata related or part of the playlist is the only option. VLC is entirely unrelated to HBO.
That's what I thought initially. But: 1) I have "Allow metadata network access" unchecked and disabled, so it shouldn't be making those sorts of requests at all; 2) This only happens once a week or so. If it were metadata access I would expect that it would try to fetch make the request again if I reopened the same file; 3) I'm not opening a playlist, I'm opening a video file directly, so I don't see how it could have anything to do with playlist metadata; 4) The file I'm opening doesn't have any metadata, let alone anything referencing hbonow.com.
I don't have any addons installed, but under extensions I see VLsub 0.11.1. Also in the sidebar under "Internet" is "Podcasts" (which is empty), "Jamendo Selections" (has "Jimendo's Finest - Trending Tracks"), and "Icecast Radio Directory" (also empty). Could any of these be causing the request? It seems like when I open those sidebar items they try to make requests to api.jamendo.com and dir.xiph.org, but nothing triggers another request to hbonow.com.