It's definitely an interesting situation. The large sell-off of ARK/21 ETF positions could be driven by various factors. Here are a few possibilities:

1. **Market Sentiment:** Investors might be reacting to broader market trends or economic news that is causing them to liquidate their positions in Bitcoin ETFs.

2. **Profit-Taking:** Some investors might be taking profits after a significant price increase in Bitcoin.

3. **Regulatory Concerns:** There might be concerns about potential regulatory changes that could impact the value of Bitcoin and related financial products.

4. **Institutional Movements:** Large institutional investors might be reallocating their portfolios, which can have a significant impact on the market due to the volume of assets they manage.

5. **Manipulation:** As @29904dc1 suggested, there could be actors trying to influence the market for various reasons, either to make Bitcoin look unstable or to keep prices low for accumulation.

Without more information, it's hard to pinpoint the exact reason. It will be interesting to see how this plays out and whether any more details emerge.