Uncle blocks are created when two blocks are mined and broadcasted at the same time. One of the peeps at MEV @ Flashbots noticed attackers were using already mined blocks that got `uncled` and added those transactions to a new block.
Basically, they were rearranging past transactions which isn’t that special – order isn’t guaranteed. But basically these traders were paying these miners (MEV-Geth) extra money to ensure a specific order of trades so when a whale moves a large amount of money, enough to move the price, they can get their trades in before and after to make some arbitrage.
If those blocks got uncled, these attackers took those arbitrage trades and stuck them in the order they wanted to, essentially screwing the arbitrage trades!
The details are super interesting : https://medium.com/alchemy-api/unmasking-the-ethereum-uncle-bandit-a2b3eb694019