Managers are often on contracts with 'renewal periods', KPIs and dismissal clauses etc.
Such contracted positions have been used in other industries (eg mining) to get 'unionists' onto 'better' terms & conditions for several reasons;
1. in the short term the conditions may actually
be better,
2. it erodes union influence,
3. if people accepting the contract are good at what's asked of them they can be promoted up the organisation, and
4. if they're not great (or really were always surplus-to-requirements) they can be KPI-failed out of the organisation in just a couple of years anyway.
Evidently, it's much easier than explaining the problem, making a case for change, and then using leadership to bring 'the people' with you.
