Reasons considered good cause could include, for example, unsafe working conditions, not being paid, a change in your job duties, discrimination, health and safety risks on the job, or some types of family emergencies.
Good cause is determined by the state unemployment office, and it varies by state. When you file for unemployment, you will be able to make a case for why you are eligible for unemployment benefits if the employer contests your claim. If your claim is denied, you should be entitled to a hearing where you can plead your case.
If you're not sure whether you're eligible, check with your state unemployment office to determine your eligibility for unemployment compensation.