The higher the rally in technology high-flyers, the louder the anxiety around a new wave of turbulence in the group.The Cboe NDX Volatility Index, a gauge of contract costs tied to the Nasdaq 100 Index, has been rising steadily this year and is now sitting near 27. The reading represents the highest level since 2002 relative to the Cboe VIX Index, which uses such contracts to measure the expected price swings for the broader S&P 500.Beneath the relatively placid surface of a broader stock market, concern is ...