As Art Cashin wrote earlier
".. the heaviest part of the selloff coincided with announcement that Macron of France would make a national address at 8:00 p.m. Paris time (around the New York close). Speculation runs to multi-week lockdown and possibly even closing the borders."
As SpotGamma details, futures have broken down to 3340 which now places the market in control of puts.
As we have discussed at length the big concern at this price level is for volatility to rapidly expand. This is because there are large Dec put positions and as the market pushes lower it may activate heavier put hedging.
This also likely makes the market more sensitive to changes in implied volatility[IV]. As volatility(ie VIX) rises the value of puts increases and additional dealer short hedges may be required (this hedge adjustment around IV changes is that "Vanna" buzzword).