They altered a state of a smart contract with consensus, although I believe this to be a mistake, they didn't roll back blocks or reorganize. Some miners used that opportunity continue a non-edited fork. I personally find this technical understanding to be distinctive from the common narrative.

I also don't think it matters and there is no consensus to do the same thing any longer.

The explanation for why they did it was because the transaction would have a very large percentage of Ether in the hands of one actor who would eventually be able to alter control of the future Proof of Stake network. If it was any other asset they wouldn't have bothered, and now much larger $ values are captured from people without any notice or fanfare. They didn't know it would be 7-9 years before Sharded Proof of Stake or the Beacon Chain would exist, and its likely that the means did justify the ends at that point in time as many of their supporters and institutional supporters had their funds in that contract.