The establishment, both Democratic and what is left of the non-trump GOP, all want to stop Sanders (and stop or coopt Warren) because he will upset the con they have been running on the rest of the Democratic party for the last thirty years or so. The scam that the establishment cares about what the voters think and will take care of you when they get to Washington. Sanders and, to a lesser extent, Warren both want to blow this scam wide open. That scares the hell out of the establishment of all stripes, which is why they are all scrambling. Both to downplay and attempt to sabotage Sanders and Warren before she started using more establishment language.
To be fair to Warren, I think she is running a con of her own on the establishment. But, I think Sanders is the right candidate at the right time to take on both Trump and the establishment. My preferred ticket would be Sanders/Warren, but that is unlikely because it is pure northeast and progressive, which is never going to make it past the “political calculus” of the VP ticket balance. My opinion is this year they should throw out that calculation and try to reach out to the 45% of non-voters in the US and get them to vote. After all, it is the largest uncommitted voting block left. Isn’t it?