A lot of people were wondering why the XVI and SVXY products collapsed so quickly even though spot volatility was not up that much and SVXY and XV were only down 15% for the day yet were down 80% the following day.

The answer is kinda subtle and most sources don't explain it well. The reason is, volatility futures trade until 4:15 but the market closes at 4:00. So this means SVXY and XIV reset at 4:15 instead of 4:00. This means the 80% termination clause is not based on the market close but actually based on the 4:15 price of volatility futures close. In the after hours in that fateful 15 minutes, the futures saw a massive surge and crossed the 80% liquidation threshold. After 4:15, SVXY and XV began to dive because at that point they were dead, but the decline was initially gradual because most people did not realize what had happened. That 15 minutes is what made all of the difference.