what's usually going on in scenarios like this is either: the subsidiary is in growth mode and needs access to public capital markets to raise large amounts for building infrastructure (and sharing the expenses with others) to compete, or what has been built is going into profitability mode and the profits are better protected/hoarded in a private company where disclosure requirements are not onerous. If this move represents the former, expect the latter in a few years, is why yahoo would maintain a large stake.
(I'm just pointing this out because there are frequently underlying reasons that have nothing to do with the type of stuff that's in the press release, "successful venture, income/exit event" etc.)