Usually managers don't join the union. Or when they become managers they are fully expected to quit the union or get demoted or take a walk. this is because they are fully expected to back the company, EVEN IF THEY ARE IN THE WRONG. Managers are salaried and make a fixed amount no matter how many hours they get.
The manager, as an agent of the employer, is expected to represent the interests of the company above the worker. The employer is interested in maximizing profits in any way possible.
The union, ideally, represents the interests of the worker even when they are not the most profitable outcome for the company.
