Because there was not sufficient liquidity for such a large trade at the going price, the trade was ultimately fulfilled, but at a 63% loss. Before the trade, that quantity of tokens was priced at around $2.28 million; however, Yearn received only around $780,000 in stablecoins because of the slippage.
Yearn quickly identified the issue and embarked on a campaign to ask nicely for the counterparties in the trade to please give some of their profits back. In on-chain messages, Yearn wrote: "one of yearns multisigs made a costly mistake last night that affected a critical source of yCRVs liquidity. we identified you as having made a profit off of this and are kindly requesting that you return as much as you see reasonable to yearns main multisig: ychad.eth. sorry we have to ask this, but hope you can understand." Doesn't hurt to ask, I guess. So far, only one wallet has taken them up on the offer, returning 2 ETH (~$4,400).