The BLM movement was completely wrong there to prevent him from speaking. Someone who wasn't even "against" their cause. The same thing happened to O'Malley as well, but he handled it wrongly, which isn't the first time he's severely mishandled public stuff.
There are going to be some serious serious issues with Hillary Clinton running honestly...
They interrupted, but they didn't stop him from speaking. That was not their intention. Sanders himself *chose* not to speak and *sent* someone from backstage to announce that he wouldn't. They're not the first protesters ever to interrupt a political event. They're also not the first protesters from the left to interrupt a democratic event. Practically every human rights or protest group has done that kind of thing over the years, to multiple candidates. This year it just happens to be the Black Lives Matter movement that has that much steam behind it. Unfortunately this became an issue exactly because Sanders himself chose not to speak.
Whatever else you say about him, when Obama was interrupted on the campaign trail (or when giving speeches), he was sure to acknowledge the protesters and if he was sympathetic (or at least wanted to be seen that way), he'd even answer or echo them from the podium. Had Sanders either let them have their 4 measly minutes of silence, or even had security remove them,
but didn't refuse to speak, this wouldn't have become as big of a deal. If clueless people in the audience weren't recorded repeating the phrase "How dare you call me racist!" it would have helped a lot as well. What Sanders has done is send a message that he may be somewhat economically progressive, but he doesn't understand or doesn't care about how anti-black racism was *intentionally* harnessed by powerful people to entrench America under the exact economic conditions
all Americans live under today. He has a partial solution, but lacks a deeper understanding of the big picture.