The biggest unfair thing was 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.

There's also the angle where the DNC was broke and basically took a huge loan from the Clinton campaign in exchange for some control of DNC's role in that primary season, but it's not clear what, if any, decisions were made differently by the DNC as a result that would have hurt Bernie.

People sometimes forget primaries are a relatively new thing and the DNC isn't legally required to do them. They have all the legal authority to pick whatever candidate they want for us to vote on. The parties run the primaries. There is no legal requirement for these primaries to even be legitimate.

The DNC literally argued in court that they had the right to do exactly that, when they were sued after the 2016 primary. They won the suit, because the judge agreed that they did in fact have that right.