It is supposedly the GBTC investors finally able to sell their locked in BTCs.
Opinion from the web:
"Grayscale was a private fund, that traded on a discount to the spot price of BTC. That means you couldn't sell your shares of Grayscale, and get the worth of BTC out. (Yes this is a function of the fact that there is no fiat liquidity in the crypto markets) Later Grayscale even stopped redemption of the funds shares at some point. Completely forgot that part.
That wasn't always the case. Back in 2017 Grayscale traded at a premium to BTC. That's why a LOT of people and institution bought into the fund.
The conversion to an ETF has been a carrot Grayscale dangled in front of their investors for years, to convince them to hold, while paying around 2% fees a year. Now the ETF is there and the investors can close their position with a very small discount. Some do it to take profits, some do it to roll into cheaper ETFs. But all will have to pay capital gains tax, further limiting the liquidity in crypto markets."