We know, in 2016, thanks to the leaked emails, that there was collusion between Clinton and the DNC to back burner Sanders.
https://www.politico.com/magazine/story/2017/11/02/clinton-brazile-hacks-2016-215774/
Does that rise to the level of "rigged"? No. But the DNC definitely put their thumb on the scale for Clinton.
In the end though, it does kind of make sense. Sanders is an Independent and only claims to be a Democrat when running for President because he knows an Independent run is a dead end.
Why would the DNC want a non-Democrat heading the party?