Nick Bilton presents more evidence of near-implosion at the International House of GIFs: "At Tumblr, the venture capitalists grew so impatient with its founder, David Karp, as he struggled to make the company profitable, they were discussing his removal before the site’s sale to Yahoo."
This jibes with earlier reports before the Yahoo! buyout that Tumblr's powers that be were looking for a Sheryl Sandberg figure—basically, a babysitter for Karp, who cares more about drones than generating revenue. This is fine if you're a drone operator in the Air Force or a Boy Scout, but not so much if you're the CEO of a business.
Now that Karp's a millionaire a couple hundred times over, would he really mind being pushed out?