No, it wasn’t.
https://www.vox.com/policy-and-politics/2017/11/14/16640082/donna-brazile-warren-bernie-sanders-democratic-primary-rigged
I’ll save you the read to get to the punchline:
“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.’ ”
Poitical parties’ hierarchies determine who they want to run ahead of time and tend to push their constituency in the direction they want. The outlier was Trump and we see how well that’s worked out, both for the GOP and the country.