Read MoreGoldman: 'Dramatic divergence' coming in market
Late in the spring, fund managers David Einhorn and Dan Loeb argued that some business sectors were experiencing bubble-like valuations and were bound to pop; more recently, in a late-July investor letter, money managers at the $25 billion hedge fund Elliott Management called the U.S. stock market "frothy."
"Financial asset prices are artificial, the equilibrium is temporary, the lack of volatility is a trap, and when the whole thing bursts, there will truly be hell to pay," wrote the Elliott managers. "Investors are 'seeking yield' now in assets of lower and lower quality, with more and more leverage, and with less and less yield to compensate for risk."
Read MorePaul Singer: 'By all measures,' US stocks 'frothy'
Despite some recent wobbliness, the S&P 500 has performed solidly so far this year, rising about 4.5 percent through July and outperforming many hedge funds, which are up an average of 3.2 percent (through June, the most recent period for which performance data is available). But July proved nettlesome for a number of fund managers, including Einhorn, Loeb and Jana Partners, all of which fell 1 percent or more during the month.
Amid the skepticism, a number of prominent hedge funds—including Ellington Management, Tilden Park Capital and Libremax Capital—are shorting baskets of junk bonds, according to people familiar with the matter.
Still, some of those bets are intended more as hedges against a drop in certain other credit-market holdings than as an outright short, say these people—suggesting that traders are still uncertain as to whether the reversals of recent weeks in both the junk-bond and the stock markets are here to stay.
Read MoreWhat do investors fear most? The Fed's bubble
"I think the market was simply ahead of itself and overdue for a correction," said one major stock-fund manager on Monday, as the blue-chip markets regained some of the ground that had been lost during the prior week. In the S&P 500, he added, the market would likely find stability at the 1,880 level, or at the 1,850 in the "worse case."
Other fund managers, including Steve Cohen, whose longtime hedge fund, SAC Capital, recently transitioned into a privately held family office called Point72, have spent much of this year expecting a significant letdown in the stock market. Whether that's occurring at the moment, of course, remains to be seen.