ARK Invest purchased 28,589 Robinhood (HOOD) shares on September 4 and an additional 27,083 shares on September 5, marking a reversal after selling off much of its Robinhood holdings over the summer. Yahoo Finance analysis suggests the purchases are driven by Robinhood's rapidly scaling event contracts, 8% month-over-month growth in total platform assets to $384 billion, and a 4% increase in margin balances to $21.5 billion in August.