While stocks increasingly move en masse due to changes in the structure of the market over the years (especially the growth of the ETF market, and HFT trading), they still vary in their movement in terms of degree.  Further, there are always the outliers who move to their own beat - although it seems far less today than in yesteryear. 

Below Bespoke Invest shows us the best and worst performing stocks in the S&P 500 year to date thru the close of trade Friday.  Keep in mind this is for a market that is essentially flat on the year - although it's been a heck of a ride getting to flat....