Global synchronous economic growth across US, Europe, Japan, EM as spurred by/in combination with accommodative monetary policy has greatly reduced volatility in the past few years. When stocks’ performance can be traced to a monetary policy implemented by the Fed, that rising tide is likely to lift all boats.
In this case, that tide has risen slowly. I think this is partly because investors have been reluctant to really jump behind this expansionary cycle/market rally after being BURNED twice in 10 years (2001, 2008). This lack of enthusiasm and low expectations for earnings and economic growth establish a low bar to clear. When the numbers turn up positive, it has slowly helped build momentum. If the numbers were “meh” or negative, investors were unsurprised and any punishment to stocks was less drastic.
Volatility tends to come back towards the end of the cycle as monetary policy becomes less accommodative and sentiment has improved. Stock performance becomes driven more by individual company earnings and you start to see more distinct winners and losers. Heading into 2018, everyone is feeling pretty good about the markets (tax reform, bitcoin, positive earnings, new highs) which makes disappointment more likely. Disappointment can contribute to brining some of the volatility back to the market. Would not be surprised to see some of these more drastic market moves in 2018.