"When the “robot trading algorithms†begin to reverse, it will not be a slow and methodical process but rather a stampede with little regard to price, valuation or fundamental measures as the exit will become very narrow.
Importantly, as prices decline it will trigger margin calls which will induce more indiscriminate selling. The forced redemption cycle will cause catastrophic spreads between the current bid and ask pricing for ETF’s. As investors are forced to dump positions to meet margin calls, the lack of buyers will form a vacuum causing rapid price declines which leave investors helpless on the sidelines watching years of capital appreciation vanish in moments."
--Lance Roberts, "The Rise of Robots, the Risk to Passive," 6/16/2017