What has the analysts uneasy isn't just the speed at which leverage is climbing, but that it's happening while the economy continues to grow.

"Leverage tends to rise most in a recession — so the fact that it is this high in a 'healthy economy' is even more concerning," the analysts wrote. In other words, they said, "mistakes are both more likely and more costly."

Corporate-bond issuance this year is on pace to exceed last year's record $1.3 trillion, data compiled by Bloomberg show. That would push sales during the past five years to more than $6 trillion. Companies that sold dollar bonds this week included Home Depot Inc., Cox Communications Inc. and TJX Cos.

Total debt at companies grew steadily at about 10 percent year-on-year since 2009 and accelerated to 16 percent year-on-year at the end of 2015.