Why do you want to be a union representative?

You can be in this job because you are truly motivated to be a union representative or just because its some job and you might be planning to shift to some other career in future. To be successful as a union representative you need the inner motivation and willingness to be able to do something for the employees in the union.
To answer this question start by telling the interviewer what you plan to achieve as an individual by being into this job and then move on to the bigger motive as a team. You can also explain how it satisfies you when you are able to help out those in need.