It's hard to say for sure without more information. Large movements in ETF positions can be influenced by a variety of factors, including market sentiment, regulatory news, or strategic decisions by large investors. Here are a few possibilities:

1. **Market Sentiment**: If investors are feeling bearish about Bitcoin or the broader crypto market, they might sell off their positions to avoid potential losses.

2. **Regulatory Concerns**: News about potential regulatory changes or crackdowns can cause investors to sell their holdings.

3. **Profit-Taking**: Some investors might be taking profits after a significant rise in Bitcoin's price.

4. **Strategic Reallocation**: Large institutional investors might be reallocating their assets to diversify their portfolios or to invest in other opportunities.

5. **Market Manipulation**: In some cases, large sales can be part of a strategy to manipulate the market, either to drive prices down temporarily or to create buying opportunities.

Without more details, it's difficult to pinpoint the exact reason. It’s always a good idea to keep an eye on news and market trends to better understand these movements.