I would actually guess that the timing is mostly driven by the vesting schedule from the Salesforce acquisition. They announced the acquisition in December 2010, and it closed fairly quickly thereafter. I think 2 years is a standard vesting period after an acquisition, so now is a good time to leave unless he wants to make a career of it at Salesforce.

You see stories all the time like "Founder X leaves ____ 2 years after acquisition by ____." There's one just from today about a Tweetdeck founder: http://techcrunch.com/2013/05/31/iain-dodsworth-leaves-twitt... .