To be fair, these are drivers themselves not wanting to work Sunday overtime shifts, which is not something the union necessarily have control over.
Not sure of the exact ins and outs of their deal, but allowing any group of workers the ability to determine their own availability on any given day in this way is a recipe for unreliability. Surely whoever signed off on such a deal must have realised that during that particular act of kicking the can down the road (of which there’s been many over the years, all intended to smooth over the dispute of the day).
There is a desire to bring Sunday into the working week, however I’m not sure drivers are going to be willing to vote for that no matter what they are offered in return.