(Bloomberg) -- The quietest week in stocks so far in 2021 has Wall Street wondering what will break the calm.Equity trading volume plunged as the S&P 500 marched to an all-time high, with the five-day average across U.S. exchanges dropping to 9.5 billion shares traded -- the lowest since October, according to Bloomberg data. Friday was particularly placid, with just 8.7 billion shares moving, the lowest daily total since Christmas Eve.The lull felt especially abrupt after 13 months of frenzied trading brought the fastest bear market ever and a furious rally not equaled in 90 years. Stuck-at-home traders turned online brokerages into casinos, while vaccine approvals in November sparked more euphoria, spurring investors into stocks they’d shunned for months. Since then, more than $575 billion has poured into the market, exceeding total inflows for the prior 12 years combined, according to Bank of America data.That all changed in April, and theories abound as to what’s behind it. The retail mania has cooled as economic restrictions eased. Stimulus bets got settled. A brief bout of selling sparked higher yields was becalmed by a chorus of Federal Reserve officials. Economic data is starting to help justify valuations. There are just fewer major issues left to drive massive market bets. No matter, say money managers, the tranquility won’t last.“We were going 100 miles an hour and now we’re back within the speed limit,” Arthur Hogan, chief market strategist at National Securities, said by phone. “We’re going to see a resurgence of volumes and volatility because this year is going to be like no other year that people have ever seen in terms of economic growth, earnings growth, inflation, a brand-new framework for the Federal Reserve.”After a 1.4% rally Monday, the S&P 500 ground out three more records to end the week as trading volumes slowed to pre-pandemic averages. The index notched a third straight weekly gain, and the Cboe Volatility Index slipped to its lowest level in 14 months. Fading bets on Fed hikes spurred the biggest weekly drop in 5-year Treasury yields since June.Traders whipsawed by the pandemic tumult are unmoved by the calm and point to signs that more turbulence is to come. Take the VIX. At 17, it’s stubbornly elevated compared to its average of 14.9 in the seven years through 2019. Bets that the summer will bring more market chaos have pushed the spread between the VIX and implied 30-day volatility four months from now to the widest level in almost nine years.Bond markets show similar expectations for fireworks -- short interest in the $14 billion iShares 20+ Year Treasury Bond exchange-traded fund as a percentage of shares outstanding rose to the highest level since 2017 this week, IHS Markit Ltd. data show, even as the ETF rallied.Meanwhile, Wall Street prognosticators think the advance that pushed S&P 500 to dot-com-era valuations is likely exhausted for the year. At an all-time high of 4,128.80, the index closed Friday ahead of the average year-end target of 4,099 from strategists tracked by Bloomberg.Skeptics have cited everything from rising yields to stretched valuation and potential tax hikes as reason for caution. Tobias Levkovich, chief U.S. equity strategist at Citigroup Inc. whose 2021 target sat at 3,800, expects the Fed to start rolling back monetary stimulus later this year and earnings guidance to weaken, posing headwinds for stocks and stoking volatility.“Sentiment is in very worrisome territory as is valuation, yet money flows continue to push indices higher,” Levkovich wrote in a note earlier this week. “Huge fiscal stimulus and supportive central banks have created the notion of there being no need to be risk averse,” he added. “Indeed, all developments are perceived as positive news. Yet, such one-sided views are not usually a good starting point.”Kim Forrest of Bokeh Capital Partners is feeling more optimistic. She expects the kickoff of what’s expected to be the best earnings season since 2018 to breathe life back into the stocks, with big lenders including JPMorgan Chase & Co. and Citigroup Inc. set to report next week. First-quarter profits from S&P 500 firms probably expanded 24%, led by carmakers, banks and retailers, data compiled by Bloomberg Intelligence show.“Unless there’s some other craziness going, like Covid, earnings always drive the market,” said Forrest, the firm’s chief investment officer. “We are heading into earnings season and the bar has been set really low, and I think the first quarter has been pretty good, so that’s encouraging.”For more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.©2021 Bloomberg L.P.