The way I see it there are only three possible reasons why he would leave a giant like Microsoft for Zynga:
1) His recent blunders forced him out and Zynga was the best he could get.
2) He's getting a huge golden parachute and he privately doesn't give a shit about Zynga as a company.
3) He wants to be a CEO and actually believes he can turn Zynga around.
It could even be a combination of the three, but I don't think there's a human being alive who could un-capsize Zynga at this point. These are the only reasons that make sense to me, though. I mean, I know Microsoft has been fucking up lately but come on, we're talking about Zynga.