Bilton's book is actually about Twitter, so he mentions the issue only in passing. Here's the line:
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.
Of course, Karp was not removed and he sold the company to Yahoo this year for $1.1 billion, proving his critics wrong. Note that Bilton says Karp's funders were discussing it, not his board. And there's a lot of interpretation about what "discussing" might mean - it could include giving him a founder/president/chief creative officer type role, while allowing new operational talent to be brought in as a CEO. That's not exactly a "removal."
We understand that the company was looking for more management bench strength at the time. Karp is known to be less interested in the nut and bolts of running a company than he is in the creative/development side. Tumblr president John Maloney, the "adult supervision" at Tumblr before the acquisition, had quit in April. (A message requesting comment from Tumblr was not immediately returned.)
There were financial reasons to wonder whether Karp was the best leader for Tumblr at the time, too. The company was losing money - down to a mere $16.6 million of the $125 million it had taken in total investment funding.
Its revenues in 2012 were paltry, somewhere between $5 million and $13 million, or as Yahoo later disclosed in an earnings announcement, not "meaningful."