MEV bots are a phenomenon that became popular in recent times: bots that use various techniques to extract value by inspecting pending blockchain transactions and then sending advantageous transactions of their own. In this case, a bot was performing a "sandwich attack": sending transactions just before and just after a pending transaction, which manipulate the price of the underlying asset, allowing the bot operator to "steal" value from the victim — "steal" in quotes, because there is some debate over whether MEV bots are really stealing, or are operating within the rules laid out for them.
In order to manipulate prices in this way, they have to put a substantial amount of money at risk. A "rogue" Ethereum validator appeared to replace some of the transactions that were being executed by the bot, leading to a loss of WBTC, USDT, Dai, and WETH totaling a bit over $25 million.