Maybe tangential, but I would have guessed people managers were not eligible to be a member of the union. Does anyone know how eligibility is determined?
The biggest advocate of pay rises and improvements to conditions is often the line level managers and those one step up from the line level managers. If this is a typical corporate job title this is a manager of line level managers. He probably has quarterly 1:1s or at least office hours supporting those at the lowest levels. It’s not like you become a line level manager, accept the 20% pay rises and suddenly change your outlook on everything. You’re never that far removed. Advocacy is important for management so I have no issues with this.
Most tech “* manager” roles where the object of management is not a person or team likely qualify.