It's the latest fad: junk bonds!  You might not even know what that is, and you certainly don't need to.  A junk bond is another term for a "high yield" bond, which is another term for a corporate bond with a credit rating that's lower than "investment grade."  We do care a bit about investment grade corporate bonds, and to be fair, it's not a bad thing to care about the high yield sector if you do so in a logical and balanced way.

It is bad to jump on a high-yield bandwagon of lamentation where every market movement gets ascribed to this latest fad.  In other words, why did bonds rally on Friday?  Many voices would say "high yield sector!"  And now today, those same voices would blame today's bond market weakness on the same thing. 

The fact is that markets don't have to have just ONE specific reason for experiencing volatility ahead of the upcoming Fed meeting.  In any event, it would be a mistake to read too much into any ONE reason.  And even then, there are probably better individual candidates for today's gyrations, including the bounce in oil and the fact that so many short positions got washed out on Friday (leaving those traders in a position to set up new shorts heading into Fed day). 

Either way, expect volatility.  Even though we know the Fed will hike, this is still a big deal, and there's plenty of uncertainty as to how market participants will trade after the fact (and apparently before the fact too!).