How do you know that? Nothing I saw or participated in was arranged outside of the official campaign and even if that were true, what difference does it make? It was still Sanders' message that inspired them.
I think how his campaign resonated with the people surprised HRC and the DNC but I don think it surprised Bernie quite as much. He has even said it wouldn't have been so successful during previous election years. He knows how angry and frustrated people are and those feelings only continue to increase - Clinton does not care and she remains out of touch with normal people.
Maybe, because these elite members of the Democratic party are so out of touch, they are just too short- sighted/narrow-minded to comprehend that Bernie, knowing a third party wasn't viable at the beginning of this primary season, was playing them by running as a Democrat, making people aware and then telling/reminding them to "Never, ever lose your sense of outrage" - after he was reminded of his agreement to endorse if she won. Personally, I think Obama and Clinton's people had to "remind" him of his agreement in a coercive manner.