As junk bonds plunge in value, many investors are wonderingwhy.

There's no obvious explanation for the 1.5 percent decline inU.S. high-yield securities in the past month, or the $9.9 billionof cash pulled from mutual funds that buy the debt. The most likelyreason is that investors are increasingly uncomfortable hangingonto bonds that are expensive by historical measures.

Chalk this one up to a collective bout of angst that looks quitedifferent from the 3.2 percent drop in speculative-grade bonds inMay and June of last year. That rout was triggered by the prospectof less Federal Reserve stimulus and, while a withdrawal ofeasy-money policies still weighs on investors' minds, that's notthe full story now.

“When Mr. Bernanke mentioned the word 'taper' last year, the world went crazy,”said Mark Lindbloom, a money manager at Western Asset ManagementCo. in Pasadena, California, referring to former Fed Chairman BenS. Bernanke. “This is a different animal.”

Some evidence that high-yield bonds aren't falling because ofrising-rate concerns can be found in investment-grade debt.Investors plowed $10.4 billion into funds focused on thosesecurities, which are more sensitive to moves in benchmark yields,according to an Aug. 4 Wells Fargo & Co. report.

Market Fuss

The question remains what to do about the selloff in junk. Everytime the bonds have lost value since the end of 2008, they'verebounded right back and then some, delivering investors annualizedreturns of 17.3 percent in the period, according to Bank of AmericaMerrill Lynch index data.

Lindbloom says he's snapping up the debt. Jefferies LLC's DavidZervos wrote in a note to investors this morning, “Maybe I havebeen looking at different data releases than the rest of themarket, but I don't see what all the fuss is about.”

On the other hand, Morgan Stanley strategists said in a reporttoday that “investors shouldn't feel pressured to buyaggressively.”

Regardless of who's right, the world's biggest economy isshowing signs of accelerating, which ostensibly would help themost-indebted companies. The U.S. will expand at a 3 percent ratenext year, faster than the 1.7 percent growth in 2014, according tothe median forecast of analysts surveyed by Bloomberg.

And even though the Fed is planning an exit strategy, U.S.central bankers keep saying they'll maintain low interest rates fora long time.

So the souring of sentiment doesn't seem to have its roots ineconomic weakness or monetary-policy concerns. More likely, FedChair Janet Yellen's comments that she's seeing some excessiverisk-taking in high yield has worried investors who are alsorattled by the fighting in Gaza and Ukraine.

Yields on junk bonds are still close to the lowest ever, andsome investors are getting out while they can relativelyeasily—before everyone exiting at once tests a market where WallStreet is using less capital to facilitate trading.

The 6.2 percent yield on junk bonds is 2.7 percentage pointsbelow their decade-long average but 3.2 percentage points more thaninvestment-grade securities, about the most since October,according to Bank of America Merrill Lynch index data.

It's nerves that are driving things now.

Some say buy. Some say sell.

Western Asset is in the buy camp, betting this rout will be likeall the others in recent years: short-lived.

“This type of behavior leads to some great opportunities for usand our investors,” Lindbloom said.

Copyright 2018 Bloomberg. All rightsreserved. This material may not be published, broadcast, rewritten,or redistributed.