Macro-funds (huge asset managers) hold equity positions and index options and individual name options. When they unwind hedged positions, there (can be) massive volume prints that can cause volatility.
On options expiration days, particularly triple and quad witching Fridays, there can be large opening and closing prints in which market-makers (block desk traders) can oftentimes being a stock to a particular level (up or down) depending on how large the closing print will be. It might be a particularly interesting day tomorrow considering next Friday is the entire Russell rebalance.
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