There's an interesting 20 minute video on Jack Bogle here. At 6:26, interviewer Jason Zweig asked whether index funds are destabilising the stock market and made stocks more volatile and making them move more in lock-step. He asked "do you think that indexing, your creation, has gotten out of hand?" Bogle's response was "The answer to that is unequivocally 'yes'".
Bogle estimated that 40% of all trading on the stock market takes place in index funds, principally ETFs, rather than in traditional funds pioneered by Bogle. He described this as "the edge of insanity" (7:29). He said that some ETFs might turn over 10,000% pa.