Superdelegates pledging for Clinton early in the primary campaign. public perception was that Clinton held a commanding lead before many states even held their votes. this could be seen as the democratic establishment choosing the candidate instead of letting the people decide in state votes. it's not literally "the DNC" but the people who were superdelegates are all party bigwigs and made up the majority of important DNC membership so it's basically the same thing. The backlash against this was so strong, it caused a rules change at the convention where Bernie lost, to prevent the same thing from happening again. Now superdelegates are awarded based on state primary results in the first round (at the convention) and then are free to vote for whomever if there's a second round.
But forget all of that what about what the courts said:
The court affirmed that the DNC and Debbie Wasserman Schultz held a palpable bias in favor Hillary Clinton.
observer.com