Something I've notice in the past few yrs trading is that large selloffs always seem to be preceded by disproportionate tanks in software stocks like ADBE, CRM, etc... To see if I could confirm and illustrate this I took the correlation coefficient of IGV (software ETF) vs. SPY and overlaid it on the SPY chart.
So it seems (at least in the past year) whenever IGV seems to underperform the S&P, a selloff ensues. Of course this could purely be coincidence, but thought it was worth mentioning given that we're seeing it creep up again on this recent rally.