Most employees in California are deemed at-will, meaning that their employer can terminate their employment at any time and for any reason. That being said, there are certain reasons that are deemed illegal, and employees are afforded protections against such actions. Illegal reasons for terminating an employee are those that are found to be for retaliatory purposes or discriminatory in nature, among others.
Discriminating against an employee is taking an adverse action regarding that employee due to their inclusion in a protected class. Protected classes under federal law include age, gender, religion and ethnicity, in addition to many others. States such as California may add even more classes to this list. Termination due to retaliation is found when an employee asserts their rights, such as equal pay or the right to a safe workplace, and the employer fires them to keep them quiet or punish them.
There are various other illegal reasons to terminate an employee. Among them are the refusal to take a polygraph test, immigration or alien status, the reporting of OSHA violations and any reasons that are found to be against public policy. Regarding public policy violations, the employer has wrongfully terminated an employee if their reasoning for doing so would be against the morals or ethics of the general public. Although this is a broad category, if an employee is terminated due to their refusal to do something illegal at the direction of their employer, being a whistleblower about company violations or asserting their rights, the termination is likely against public policy.
It is important for employees and employers to be aware of these illegal reasons to terminate an employee. Although they are typically classified as at will, employers are not allowed to fire an employee for any reason if they violate the employee's protected rights in doing so.
Source: Findlaw, "Wrongful Termination Laws: Illegal Reasons," accessed on April 12, 2015