Grayscale may have sold a large amount of ETH when the ETH spot ETF was approved and Mt Gox paid compensation. There may be many reasons. Here are some possible reasons:

Profit-taking: After the good news comes out, the price of ETH may rise. Grayscale may think that selling at this time can make a good profit, so it chose to sell.

Market expectation adjustment: Although the approval of the ETH spot ETF is regarded as a positive, its actual impact is difficult to predict. Grayscale may think that the price of ETH will not rise sharply in the short term, so it chooses to sell.

Risk management: Grayscale may be worried that market volatility, regulatory changes or other unforeseen factors will affect its portfolio, so it chooses to sell ETH at the right time.

Portfolio adjustment: Grayscale may need to rearrange its portfolio in accordance with established strategies and goals. Selling ETH may be to make the portfolio more balanced or to invest funds in assets they think are more promising.

Responding to redemption needs: Grayscale's products, such as the Grayscale Ethereum Trust, may face redemption demands from investors. In order to meet these demands, Grayscale needs to sell some ETH.

Mt Gox payout: The Mt Gox payout will bring a large amount of ETH into the market, which may cause the price to fall. Grayscale may have foreseen this and chose to sell some ETH before the payout to avoid excessive losses.

It should be noted that Grayscale's specific decision-making reasons are the result of a combination of factors. The market environment is constantly changing, and investor behavior will also affect prices. To predict the future trend of ETH prices, it is also necessary to consider multiple factors such as market supply and demand, overall economic conditions, and regulatory policies.