I dont see anything shocking in that. Bernie was basically an independent that ran as a Democrat out of convenience and therefore the Dem leadership were not too fond of him.
Of course, they didn't look at it this way when they were counting his vote and taking it for granted in the House or the Senate....
And also - the problem isn't BERNIE, it's the fact Hillary has to win over his supporters. The Sanders supporters can be broken down into different groups: people who hate the establishment (would they vote Trump, I don't know), people who saw him as the lone vehicle against Hillary and therefore voted for him (will Trump get these voters or will they stay home), some millennial socialists (Hillary has most of them - I think) or theoriticians, and some who hate the Establishment but COULD NEVER vote for Trump (the Don has no prayer at getting them), and maybe 1-2 other terms.
I'm usually VERY GOOD at projecting this stuff. I figured Kasich was the VP pick (and stories are saying Trump was offering the man more than anyone ever has in terms of power) and Kaine for Hillary, but I'm also one of the few who projected Sarah Palin for McCain back in 2008 and one of the few who kept telling folks the Hillary-Rudi election for Prez would never happen. Most of the time I can pick it up.
But I can't this time. I thought Trump would win early if he was lucky (with the 10-way or whatever it was split), the party would rally behind Rubio or Kasich to stop him and make the others get out, and like Jesse Jackson did with Dukakis, Trump would then get creamed the next several weeks and be done. I also figured Bernie would cause Hillary some trouble but still lose.
My inclination right now is a Hillary win simply because of the electoral problem, but the selection of Kaine and this news story has to tick off some of the so-called 'true believers' in the party.
Will some/many of them be angry enough to say, "Well Trump isn't really all that conservative plus the Congress will stop most of his nonsense, so it's worth a risk and someone else in four years?" I honestly don't know. It defies everything I know about political science that Trump could even win the nomination so don't get me to lying about it.