From what I understand:
Two women from a group called Outside Agitators 206 in Seattle decided to crash a Medicare/Social Security rally on behalf of #BlackLivesMatters, interrupting a speech by Bernie Sanders he was invited to give. The event organizers initially said they'd let them speak after Bernie, whose time was limited (he had his own rally to get to). The women were adamant, promising they would "shut it down" if they didn't get to speak right then and there. Bernie spoke to the main organizer and gave them the opportunity to say what they wanted to.
They covered topics of police brutality and gentrification in Seattle as well as calling the audience "white supremacist liberals," to much chagrin, and then they demanded 4.5 minutes of silence for the hours Michael Brown spent uncovered in the street (this was the 1 year anniversary of the shooting). The crowd was a mixture of annoyed yet compassionate, ultimately doing as they were told. When Bernie approached the podium, the women screamed at Bernie's face, giving him no chance at any reasonable dialog. Seeing that he was already going to be late for his next rally, and how completely unreasonable the women were being, he had no choice but to leave, which he did.
People are saying he "should've done more" but I don't know what. It was a lose-lose situation.