Hallelujah, high-frequency traders didn’t short the market that much :
Our analysis of trading on August 24, 2015, shows that while there were bursts of aggressive HFT activity during the sell-off, it was the institutional activity, not the HFT activity that led and dominated the sell-off. Specifically, the events have appeared to unfold as follows: institutions would sell particular securities, creating acute selling pressure in the markets. Aggressive HFTs would then step in and sell the market further, but only for a relatively short period of time.
Abnormal Capitalist 