High levels of corporate debt are suddenly a whole lot more worrisome than they were just a couple of months ago.

U.S. financial account figures from the Federal Reserve released Friday showed the amount of money U.S. companies have borrowed continues to swell. Domestic nonfinancial companies had $9.95 trillion in debt outstanding in the second quarter, an increase of $1.2 trillion from just two years ago. At 47% of gross domestic product, the level of corporate debt in relation to the economy has never been so high.

That...