Sanders himself was frantically trying to get them to stop. He sent a text and an email to delegates asking them not to protest from the floor:
"Our credibility as a movement will be damaged by booing, turning of backs, walking out or other similar displays," Sanders wrote.
Some delegates either didn’t get the message or didn’t care. Over the past few months, Sanders had been energizing his supporters by telling them that victory was possible, even after it seemed mathematically highly unlikely for him to win the nomination. And at least one angry delegate said he took that message to heart:
Iowa delegate @chris_laursen: "Bernie basically fed us a bunch of Mountain Dew and now he wants us to go to bed. It’s not going to happen."— Jason Noble (@jasonnobleDMR) July 25, 2016