Donna Brazile, the former interim chair of the Democratic National Committee, has a new book out called Hacks: The Inside Story of the Break-ins and Breakdowns That Put Donald Trump In The White House, and in it she describes the secret dealings within the DNC that basically secured Hillary's position as the party's nominee for President way before she won the nomination.
To promote the book, Donna has written about her findings in Politico:
The agreement—signed by Amy Dacey, the former CEO of the DNC, and Robby Mook with a copy to Marc Elias—specified that in exchange for raising money and investing in the DNC, Hillary would control the party’s finances, strategy, and all the money raised. Her campaign had the right of refusal of who would be the party communications director, and it would make final decisions on all the other staff. The DNC also was required to consult with the campaign about all other staffing, budgeting, data, analytics, and mailings.
When the party chooses the nominee, the custom is that the candidate’s team starts to exercise more control over the party. If the party has an incumbent candidate, as was the case with Clinton in 1996 or Obama in 2012, this kind of arrangement is seamless because the party already is under the control of the president. When you have an open contest without an incumbent and competitive primaries, the party comes under the candidate’s control only after the nominee is certain. When I was manager of Al Gore’s campaign in 2000, we started inserting our people into the DNC in June. This victory fund agreement, however, had been signed in August 2015, just four months after Hillary announced her candidacy and nearly a year before she officially had the nomination.