Sanders styles himself as the leader of a revolution. Revolutionaries use the weapons they have at their disposal. If she proved difficult to handle, or didn't know her place in the pecking order, she could have been discarded if necessary. However, climbers like her want to attach themselves to people with power. If Sanders had effectively used what leverage and power he had, and it looked like he was going to be President, Gabbard would have kept her place in line. She would have wanted rewards for her loyalty, and she should have been rewarded accordingly for doing her duty.
This is the problem. Sanders never used any of his influence or power at all. He was--and is--a very weak leader. Yes, he says all the right things. He presses all the right buttons, but he has always lacked the ability to truly deliver. Now we are watching his campaign crash and burn as a result.
Gabbard is jumping ship from the DNC and officially attaching herself to Sanders now, because she knows she has zero chance with Clinton. That bridge is already burned, the Clinton's do not forget the type of insubordination that she showed within DNC. Therefore, she hopes to create for herself some type of position as an outsider if Hillary takes the Oval Office. She is likely eyeing a Grayson type position in the party--hoping to become a liberal truth teller popular with the base. She is hoping to cash in what she can at this moment before it is too late.