This article is old, but a good read: https://www.vox.com/policy-and-politics/2017/11/14/16640082/donna-brazile-warren-bernie-sanders-democratic-primary-rigged

The 2016 Democratic primary wasn’t rigged by the DNC, and it certainly wasn’t rigged against Sanders. But Democratic elites did try to make Clinton’s nomination as inevitable, as preordained, as possible. And the party is still managing the resentment that engendered in voters. “Once somebody doesn’t trust you,” sighs Buckley, the New Hampshire Democratic chair, “it’s very hard to get that trust back.”

The DNC did mess with the 2016 preselections, but the article argues that that probably benefited Sanders, by giving him much more of a platform than he would otherwise have had.

But it also seems like a footgun for the democrats as a whole, and probably contributed to the 2016 anti-establishment vibe that led to the Trump win