High-yield (or so-called junk) bonds are set to experience their first annual loss since the recent credit crisis. Because of the higher risk involved with these borrowers, they tend to be the first to experience trouble. Thus, many investors believe that they represent a leading indicator for overall market performance. If this is true, it could indicate trouble to come. See article here: WSJ.
Dolvin, Steven D., "Are Junk Bonds Signaling Trouble Ahead?" (2015). All Chapters. 146.