It's a cluster of factors - the primary one being a lack of a bench of backup pilots - compounded by layoffs during COVID and now an inability to ramp up training to get pilots re-certified and re-hired: https://www.youtube.com/watch?v=kO39nIcuPhQ

COVID vaccine shenanigans with a large chunk of the workforce near retirement age and just saying "fuck your mandate, I'll retire early" is just the cherry on top.