Technology has been the best-performing sector this year, advancing more than 20 percent in that time period. Financials, meanwhile, have spiked nearly 5 percent over the past three months after a slow start to 2017.
"Historically, tech and financials trade together. You could be seeing that correlation come back," said Mike Bailey, director of research at FBB Capital Partners. "I think the short-term is increasingly uncertain without too many positive catalysts out there."
Wall Street also worried that stocks could end the summer on a sour note. The S&P and the Dow notched their longest weekly losing streak since May on Friday, as Wall Street navigates a part of the calendar that's typically bearish for stocks.
According to data from Bespoke Investment Group, the S&P 500 has posted a median return of 0.04 percent in the two weeks between Aug. 21 and Sept. 9 over the past 10 years. During the past two years, however, the S&P has declined 0.2 percent and 2.5 percent, respectively.
"We think we'll get a small sell-off in the short-term, maybe between 2 and 5 percent, as volatility picks up," said Eric Aanes, president and founder of Titus Wealth Management. But "we think that could be an opportunity to buy."
Stocks have pulled back from record highs this month, with the three major indexes falling at least 1 percent in the period. And while the Dow, S&P and Nasdaq are still at least 2.3 percent off their all-time highs, the market is showing other signs of cracking.
Sam Stovall, chief investment strategist at CFRA Research, said in a note to clients Monday that less than 30 percent of S&P 1500 sub-industries are trading above their 50-day moving average, a key technical level. The S&P 1500 combines components from the large-cap S&P 500, S&P Mid Cap 400 and S&P Small Cap 600.
"This level is typically a threshold below which investors begin to turn more hopeful of a market turnaround. Hope doesn't always prevail, however. Not surprisingly, the declines in excess of 5% were associated with the deeper drops in sub-industry participation," Stovall said.
Investors also paid attention to geopolitics on Monday. President Donald Trump is set to lay out a U.S. strategy for the war in Afghanistan and engagement in the South Asia region. The U.S. and South Korea are also set to kick off war games on Monday.
"We have a few geopolitical issues hanging over the market despite a quiet weekend," said Quincy Krosby, chief market strategist at Prudential Financial. "While the war games are nothing new, the will have more importance because of everything that's happened with North Korea."