So what would China's motivation be here?
Might be just a crew paid off by Russians to do it.
In my country saboteurs largely weren't Russian - it's easier to pay off a local than have ano5 Russian cross the border, when his predecessor gets caught.
China has a lot of interest in the war not ending one way or the other. Their peer competitors are spending resources on it and a potentially problematic regional competitor is becoming more irrelevant the longer it runs.
Finding out how far they can go without consequences / test the will of another nation to do something?
Article indicates this isn’t the first time.
"Chinese-flagged" does not equal "Chinese operated"
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China likely has nothing to do with this. It is unlikely they have any participation or even knowledge of this. Twice now some Russians in a China flagged ship caused trouble, and the China-flagging seems very intentional.
Russia is desperately trying to make the China-Russia thing a reality, and is probably trying to drag them in against their great resistance. China has zero credible reason to be dragged into Russia's nonsense, and a billion reasons why they want nothing to do with it.
The ideal outcome of this is that China realizes that Russia is outright trying to drag them into conflict, and that they repudiate that country entirely.