Well they also have a significant share of invested funds ($5.3 trillion under management!), it would be really weird if they didn't have a significant share of many companies.
Someone with index funds has no choice: has to dump the good and the bad.
https://en.wikipedia.org/wiki/Disposition_effect
Regardless, my point is smart stock pickers will come in and bid up companies that get oversold. No damage done unless you tried to time the market in your IRA and sold at the bottom. People who continue automatic purchases of broad indexes will be happy (at some point!) for the discount.
But they can be rebalancing instead of going to cash as well; Such as moving from SPY (S&P 500) to SPYD (S&P 500 dividend stocks)