To the greatest degree of our understanding, the mid-career slump happens to some people because priorities can shift mid-career. Kids/family starts to grab more attention, seeing life as more than a job and wanting to live it to the fullest becomes more prevalent as one becomes more aware of the clock ticking, for professionals they've likely started to build a bit of a nest egg so money doesn't seem as important before, etc. They slump because they just don't care about work like they used to.
How does your union keep people focused on the job? Or is it simply that the union self-selects those who are already focused on job-related matters and wouldn't end up in the slump anyway?