The combination of HFTs and new VIX related ETF/ETN products created the tools for this market collapse to transpire. It's become too easy for these products and techniques to trade hand in hand linked to one another.
Here I am, a 45 year market veteran, and named to the ETF Data Base ETF Hall of Fame, but was highly critical of these products when matched together. Especially the leveraged ETNs when paired with HFTs created accidents waiting to happen.
The SEC didn't do a good job evaluating how these products could, and have, upset orderly trading markets. It's a disgrace.