My take on the 2016 primaries is that both things are true. Bernie lost fair and square....but many of Clinton's surrogates weren't willing to take the chance again. John Oliver reported that in Nevada, they changed the rules and backdated them. Basically saying "Sorry Bernie, we decided just now that the deadline to do that was three weeks ago."
Sanders would have lost anyways, but it wasn't a good look for Clinton to have her people doing stuff like that. Even if she wasn't directly involved (State party politics has its own stink much of the time) it reflected poorly on her.