There are rumors that it was still not a mistake and the story is related to extortion and fraud - it was not a single transaction and in General it all looks very strange and stupid... Apparently, the case here is not as simple as it seemed initially.
Yeah, that was Vitalik's assumption, i.e. attacks of hackers on exchange the aim being to blackmail it. In that guess what I don’t understand is" the partial access to exchange key". How it is possible to have the capacity for sending transaction with any fee and at the same time to be incapable to withdraw fund with the normal fee. I must have gotten Vitalik's thoughts mixed.
Although I can’t compare myself to Vitalik but looks like his "blackmail" assumption doesn't up to scrutiny. Reputable blockchain researchers expressed doubts that is basically consistent with mine. The stirring transactions remain an enigma so far.