In the implied scenario, it applies when the job terms change unacceptably after you’ve accepted its responsibilities. They can’t expect you to quit after pulling a form of bait-and-switch. In this case, she had no problem issuing marriage licenses for actual marriages, but since SCOTUS redefined “marriage” such that the KY definition is eviscerated unto meaningless, she understandably is refusing to issue ANY “marriage” licenses because (A) certain applicants are demanding what she cannot in good conscience condone/certify, and (B) the term is now legally so meaningless in that jurisdiction that there is no bounds for what was precisely defined & established to bound. She acted properly: refusing to act on that which has been rendered undefined in law and immoral in conscience.
Bear in mind that she was elected to office less than a year ago. At that time there was widespread speculation in the media and among the general population that the Supreme Court would legalize same-sex marriage. Obergefell had already been in the court system for a year, and several state bans on it had already been overturned in the courts. She likely would have been aware at the time she chose to run for office that she might well be expected to issue a license to a gay couple somewhere in the future. You can’t say that she got blindsided by this.