A QuickTime backed MediaDocument falls back to a PluginDocument if a media file contains one or more "unsupported" track types. The list of supported types does not include some fairly common types that are not audio, video, or text/caption, but which are not rendered at all so should not cause fallback. These types are: time code, time code 64, odsm, and sdsm.
<rdar://6844702>
http://trac.webkit.org/changeset/43109