It's a chicken egg thing. Both buying vol and selling equities are risk-off trades.
First VIX was very low and equities at all time high, so people start buying some vol... I think it started by buying OTM calls in SPX, pushing up VIX.
Then some started selling equities, because we're at all time highs and 10 yr yield creeping up.
Then vols rise a bit further and together with the selling pressure possibly some short vols get squeezed... snowballing into more vol buying and equity selling since vol is getting expensive and equities still at relatively high levels... heavy selling turns into heavy vol buying... vol derivatives are being squeezed... VIX spikes and all kinds of algo's run for the door... crash...