Complete horseshit smear tactics, but a lot of people will fall for it. Finally the establishment has something that might stick, god knows they are trying relentlessly on both sides.
Basicly they made a deal for 30k base pay for a full time position (above 15 dollars/h). Some workers decided to work more, which meant their hourly pay went down cause of the flat rate. Then the campaign/Bernie came in and said they would raise wages for everyone so they still earned at least 15$, but the union rejected it based on the original deal. Now the campaign is negotiating with the union.
That's the story.
How ironic that the entire reason why this is even worth mentioning is because they had such a good deal in the first place. Sanders campaign get the best pay, healthcare without premiums, mental benefits, parental leave, gascard and more. No one else is getting this.