California is an at-will state. Thus, your employer can fire you at any time and for any reason, just as you may leave your job at any time and for any reason, barring a contract that specifies otherwise.
However, the glaring exception to this rule occurs when a termination violates state and/or federal anti-discrimination and whistleblower protection laws. Firing an employee because of being in a protected class or because of reporting a legal or health and safety violation may be considered wrongful termination and open the employer to complaints and lawsuits.
Defining constructive discharge
Some employers seek to avoid accusations of wrongful termination by trying to make things so unpleasant for targeted employees that they leave of their own "free will." The law calls such actions "constructive discharge," meaning that although it is not technically a discharge, the law will consider it equivalently. An employee who was the victim of constructive discharge may have a cause of action for wrongful termination.
When do working conditions become intolerable?
Constructive discharge can be challenging to prove. California law requires the employer's actions to rise to the level of creating intolerable working conditions. Many workplaces may be difficult or unpleasant without being intolerable. Some examples of unacceptable behavior may include threats, constant yelling, humiliation and other types of behavior that most reasonable people would consider impossible to work with. Whether a condition is intolerable may also depend on specific circumstances. The basic tenet remains that the conduct must make it virtually impossible, not merely inconvenient, for the employee to continue to work for this employer.
If the intolerable behavior comes from other employees or supervisors, the employee may also need to show the employer knew about them and did nothing. Courts typically also require the employee to show that the bad conduct continued to occur until the time the employee left. Documenting negative interactions can help put together a timeline of events that would bolster the case.