It may be a mistake. That's why it is better for the freelancers truly to be doing their own background checks before they agree to work with clients. When a client is fired by the previous freelancers, to be save, you need to make sure that you prevent the same occurrence. For instance, you may decide to talk about money first to see the reaction from him. Ask all what you believe can be the cause of disagreement. What do you think about this?