Fresh concerns about China's economy and broader emerging markets have widely been named the primary culprit for the selloff. Yet the Russell 2000 small-cap index, which is often considered to be less levered to global growth, fell more 4 percent in the first half-hour of trading as well, and was off more than the S&P at points in the morning.
Neil Azous of Rareview Macro said although small-cap stocks offer liquidity in market dislocation or a mispricing of assets, they are affected by global growth or emerging market issues that could increase the probability of a U.S. recession.
"Small caps are not immune to that new recession discount rate and that can outweigh the prospect of low interest rates and low input costs, such as crude oil," he said.
Andrew Burkly of Oppenheimer said investors will continue to flood out of small-cap stocks faster than large-cap stocks under heightened volatility.
Read More Falling market tide 'lowering all boats'
"You'll definitely get more risk-off behavior from small caps today," Burkly said in a Monday phone interview. "As long as there's this risk-off and volatility, it's going to hurt the small caps more than large or mid."
The CBOE Volatility Index, also known as an investor "fear gauge," spiked to levels not seen since February 2009 on Monday, rising as much as 90 percent.
The Russell 2000 entered correction territory Friday, which is technically defined as falling 10 percent from a 52-week high, and dropped deeper Monday.
According to Michael Block of Rhino Trading Partners, the major moves in the Russell 2000 are being driven by several factors. He said the index is more susceptible than major indexes to market swings and slowing global growth.
He also said investors have increasingly been using small-cap stocks and the Russell 2000 ETF (IWN) to hedge against high-yield bonds. Since credit risk in so-called junk bonds is difficult to hedge, investors may have bet against the Russell as a way to manage that risk.
As high-yield bonds are dealt a blow from investor nervousness about growth and energy, small caps will tumble with them, Block said.
Read More 10-year bond yields break below 2%
"The correlation has become unquestionable here. There are a lot of folks mucking around in the Russell that were not doing so three or four years ago," he said.