As I understood, it's this Grayscale ETHE ETF that existed before but was not spot ETF and you could only buy without ever redeem the ETH behind it, the only way to sell was to find a new buyer for the ETF, this drag down the price to 50% of it's supose ETH value. When ETF spot was appoved, this Grayscale ETHE ETF was converted to spot with possibility to redeem the ETH 1 to 1 in equivalebt dollar value (you cannot get the real ETH). So a lot of people locked in before, including a lot of speculator that bought it at a discount to make a profit once the ETF spot happen, massively redeem the ETH value in USD forcing Grayscale to sold them.

The exact same happened to the Grayscale Bitcoin ETF, except that the price dump was temporary and absorbed quickly by the huge demand for BTC.

At some point, like for the BTCE (bot sure of the name), once everyone who wanted to sell sold, ot will get more bullish again, this could happen super fast... or never. I make the bet that demand will rise one last time pushed by the next BTC rally, but maybe I'm wrong and will just sell my ETH at an ever lower BTC price.
I might also keep some fiat in hope that BTC deep a bit after ATH but that's even more risky.