This is a very common interview question, especially in social service work. The interviewer does not need to listen to you bash about the old company you worked for- you never know who knows who. If you did have a bad experience at your last company, you can share the experience but keep if general and do not use names of people that you had issues with.

Sometimes people switch jobs because they need change in their work life and want to grow more professional. There are many reasons to leave jobs, so don't feel unfortunately discussing your reasons, just remember not to bash on your past place of work.