A recent article in Planet Yelnick entitled "The Herd Leaves the Market to the 'Bots" is well worth reading. To wit:

The WSJ noted two odd phenomena in this market: small investors have largely bailed out, and (perhaps as a consequence) individual stocks in the S&P 500 have been tracking the index itself to a degree not seen since the 1987 crash (see Chart 1 below).

The average correlation is 44% (since 1980), but is now 81%, higher than in the crash of 2008 (79%) and approaching the all-time high in 1987 (83%). The conclusion from the correlation: fear drives investors to treat the market as a homogenous entity. Adding in the exodus of retail traders, this time the implication is the market has been left for the index 'bots.

Chart: US Stocks' correlation to the S&P500 index (Source: WSJ)

The article is quite technical, with a fair bit of reference to Elliot Waves counts. If you have the patience, I would suggest that you read the article carefully. The writer opined that "...the high correlation is an extreme example of herding behavior..." and in the worst case scenario, "...the floor can drop out of this market rapidly...". Now, that's discomforting.