Alibaba forced Yahoo to sell a sizable part of their position against their will, under threat of what might happen otherwise.
Then Alibaba, specifically Jack Ma, stole roughly $30 billion from Yahoo and Softbank by removing Ant Financial (and directly transferring ownership to himself [1]) from Alibaba without proper compensation to Yahoo and Softbank. You can't do anything about that sort of extraordinary theft in China, so that was that. And now about half or more of Ma's wealth is made up of what he stole from Yahoo and Softbank shareholders in that maneuver (out of his $40 billion, roughly $20-$21 is Alibaba, the rest is Ant Financial).
[1] per Bloomberg: "Ma controls Ant Financial through two China-based holding companies, Jun Han and Jun Ao, which own 42 percent and 34 percent of Ant Financial respectively, according to Alibaba's 2015 annual report."