Gradual deterioration in the quality of corporate debt traded in the markets has been quite spectacular over 2018:
The above chart shows that at the end of 3Q 2018, the market share of BBB and lower-rated corporate credit is now in excess of 50%, in excess of USD4.4 trillion, matching prior historical record set at 4Q 2017-1Q 2018. BIS' Claudio Borio was quick out on the rising risks: https://www.bis.org/publ/qtrpdf/r_qt1812_ontherecord.htm, saying "...the bulge of BBB corporate debt, just above junk status, hovers like a dark cloud over investors. Should this debt be downgraded if and when the economy weakened, it is bound to put substantial pressure on a market that is already quite illiquid and, in the process, to generate broader waves. ... What does this all mean for the prospects ahead? It means that the market tensions we saw during this quarter were not an isolated event. ... Faced with unprecedented initial conditions - extraordinarily low interest rates, bloated central bank balance sheets and high global indebtedness, both private and public - monetary policy normalisation was bound to be challenging especially in light of trade tensions and political uncertainty. The recent bump is likely to be just one in a series."
You can read my views on the latter aspect of the markets dynamics in the post that will follow.