I took a closer look at the GBTC chart vs. BTCUSD and noticed the GBTC premium has been falling quite a bit recently. It's dropped in half since February alongside these big inflows.

I think there is increasing pressure to arbitrage the GBTC price. Even with the 1-year lockup period, a 30% premium (that's what it was at the Febuary peak) is just ridiculous in a market with increasing regulated options at lower carry costs.

In other words, institutions could short GBTC while it's trading at a huge premium while buying shares at a discount (the NAV price) during private placements.

If this explains the increased volume, then we can probably expect the volume to die down (and maybe Grayscale's overall holdings to decrease as well) once the premium is no longer worth arbitraging.

https://cointelegraph.com/news/what-does-grayscales-gbtc-falling-premium-tell-us-about-bitcoin-price

I've been struggling to find an explanation since GBTC is so unattractive as an investment with its ridiculous premiums and 2% management fee. This is the best I can come up with.