One key group of stocks is getting left behind this year–and that has some worried about the market

The Russell 2000 is badly underperforming the S&P 500, and investors are pulling money out of the IWM, the ETF that tracks the index, at a heady clip.The iShares Russell 2000 ETF saw $1.8 billion worth of redemptions in the week ended May 24th, according to...