Here's a quote from the book after Ev was told he was out with a vicious quote from Fred:
Williams, stunned, picked up the phone and began dialing. Bijan Sabet was apologetic and insisted that they wanted to keep him on in a product-advisory role. According to several people at the company, Fred Wilson, however, said he thought Williams had always been a terrible C.E.O. “I never considered you a founder,” he said. “Jack founded Twitter.”
Other portfolio investments of Fred's have followed a similar pattern of having the original CEO pushed out once they get to a certain level of success.