A "Quite Extraordinary Day" In Gold - What Happened?

Goldman Sachs trader, Adam Gillard pointed out that yesterday felt similar to 21st October sell-off (albeit more extreme), where he thinks the vol move from a large short-dated GLD block triggered model liquidation, which was exacerbated by high-frequency market-makers (on GLD).

The salient point around the October sell-off was that the GLD did not cut despite then record volume.