Nobody is declining to work Sundays to cause any chaos. They are declining to work them because they want a day off, it's that simple. If an organised effort or instruction was made to collectively refuse to work Sundays that would have to have gone through the required processes at the relevant union or otherwise that union would be taken to the cleaners.
A lot of drivers want Sundays off because they are working extra days during the week - and working an extra day during the week pays 50% more than working an extra day on a Sunday. It doesn't take a genius to work out why they are then short on Sundays.
Why are drivers working so many extra days during the week? To cover training. Even with dozens of extra drivers working hundreds of extra days during the week there are still cancellations all through the week.
The situation was attempted to be addressed earlier this year, however the management saw it as a golden opportunity to rip up all working agreements wholesale rather than address the minimum number of agreements necessary to bring Sundays inside the week on the west side of the business. This was quite rightly declined by the drivers. Even if it had been accepted it would've meant continued disruption for years - bringing Sundays inside the working week for 650+ drivers who would normally work Sundays as overtime would mean recruitment of about 15% more drivers - call that 100 extra drivers. It would take a minimum of three months (more likely six months) to recruit those drivers. Another twelve months or more to train them and get them passed out as competent drivers with basic routes and traction, then another one to four years before they are fully competent in all routes and traction. You would need to recruit extra driver instructors, driver trainers and driver managers.
At say Buxton who have only about 30 drivers and sign very few routes and traction that might be possible in six months, but of the 100 total drivers required to be recruited you'd only need one or two for Buxton if any - there is a waiting list of drivers wanting internal transfers, which means there will be replacements needed at Piccadilly or whatever depot those drivers transfer from to go to Buxton. The same applies at Workington which is another tiny depot. At each of Manchester Victoria and Piccadilly there are 200 drivers and already thousands of days training outstanding. Wigan, Liverpool and Blackpool have just under 100 each. Blackburn has 55, I'm not sure how many Barrow have.
The easy way to get Sundays covered at no extra cost to the company would be to ban rest-day work during the week. However that really wouldn't bode well for the other six days in the week.
What about the fact that working a rest day from Monday to Saturday pays so much more than working a Sunday? Drivers rightly want a decent rate of pay to give up their own time off, especially as working a shift as a driver often impacts on either the day before (making sure you're well prepared for that 2am alarm clock) or the day after (getting home at say 3am) too. Northern are not in a position to be able to play hardball with the rate of pay for working a rest day during the week - there is so much rest day working required, and drivers obviously have the right to decide whether they want to work extra days of not. Bringing the rate of pay for Sundays up to make it more attractive to work Sundays is one option, but that costs money - if rest day work somehow no longer became required, the Sunday rate of pay would remain. This could end up costing the company more eventually if it was to bring Sundays inside the working week as drivers would then want the correct recompense for the pay they would lose by not having Sunday pay as extra. Even if the pay was made the same for Sundays as for working extra days during the week some drivers would decline as they want the time off with their family when their family is actually around.
TL;DR - drivers would rather work extra during the week than Sundays.