Bank of America's chief executive officer, Brian Moynihan, has projected that third-quarter trading revenue will be "relatively flat" compared to the same period last year, a forecast that starkly contrasts with the surging activity seen on Wall Street during the first half of the year. The cautious outlook sent shares of major banks into a sharp decline on Monday.

Speaking at a conference hosted by Barclays, Moynihan indicated that investment banking fees are expected to land between $1.6 billion and $1.8 billion. This guidance fell well short of the roughly $2 billion that analysts had been anticipating, triggering a sell-off in the sector.

Following the announcement, shares of Bank of America Corp (NYSE: BAC) tumbled as much as 4.4% during the session. The negative sentiment also dragged down rivals, including Goldman Sachs Group Inc (NYSE: GS), whose stock also moved lower amid fears that the bumper trading environment may be cooling off.

The forecast comes after a wild and highly profitable first half for Wall Street traders, during which Bank of America's equity trading revenue skyrocketed by 70% in the second quarter, reaching record levels. The stark contrast between that historic performance and the current, more subdued outlook has rattled investor confidence in the sustainability of big banks' earnings momentum.