So apparently, Roy Bostock is on the way out, and a source familiar with the situation had a good answer as to my question of why it hasn’t happened yet.

For one thing, as Kara reported, I’m hearing that Jerry Yang’s sudden decision to go was his own, and that it was very hastily done. The speculation from insiders is that he had no interest in being the prototypical Valley founder who stays too long and is ousted by Wall Street.

Bostock has a ticking doomsday clock hanging over his head too, we hear. Activist shareholder Dan Loeb has the opportunity to present a new slate of directors on February 24, which gives Yahoo’s board a little more than a month before it’s under direct threat.

Until then Bostock needs to stick around. Why? To close the incredibly complex three-way deal involving Yahoo’s Asian assets. Bostock is one of the main architects of the deal and as much as new blood is welcome right now operationally, there needs to be some consistency to see the deal through. Bostock has overseen a lot of screw ups at Yahoo, but letting this deal fall apart would enrage shareholders even more…if that’s even possible.

Yang’s departure clears another hurdle: Yahoo’s Asian assets were one of his most proud accomplishments as a founder and board member and he had the same emotional regret about letting them go as he had towards selling the company back in 2008.

As always with board politics there’s ego involved here too. Once the deal unlocks some $17 billion in value and the stock pops up, Bostock and the others can leave on a high note and still avoid the nasty proxy battle to come. And Bostock has reportedly been given the luxury of leaving on his own terms, like Yang. Leaving then, versus now makes a big difference to Bostock’s reputation.

But can the Asian deal come together in time? Given the complexity, it’ll be a photo finish, sources say.

It’s possible that a board shakeup was an unspoken term in new CEO Scott Thompson agreeing to take the job, or at least something that he knew was coming. As we’ve written before, the Yahoo board has a special knack for second-guessing and interfering and making any CEO look horrible. We know for a fact that potential new board candidates were already being approached just days after Thompson’s appointment was announced.

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