The $9.8 trillion U.S. corporate-bond market may look pretty sleepy right now, but there's more and more happening in its shadows.

Instead of bothering with trading investment-grade bonds themselves, investors are increasingly turning to derivatives tied to the creditworthiness of specific companies. Volumes in such synthetic wagers have surged to the highest levels since at least the beginning of 2011, in many cases outpacing trading in the underlying bonds, according to Barclays Plc data.

This reflects investors' concern that they can't get in and out quickly enough in the market for cash bonds. Wall Street is pulling back in debt trading, potentially leaving investors vulnerable at a time when the outlook may change quickly, given that the Federal Reserve is grappling with how to exit from a sixth year of record stimulus.