Mitigation of damages issues arise when the employee who was wrongfully terminated gets a new job and then is terminated by the new employer. Many times, while the wrongful termination lawsuit is pending the former employee finds new employment. Some times, the employee gets terminated by the subsequent employer. When this happens the defense lawyer always argues that the back pay is cut off by the subsequent termination. That may or may not be true.
It depends on the circumstances that caused the subsequent termination. The defendant has the burden of proving that the employee's conduct that caused the termination from the subsequent employer was not motivated by unreasonable working conditions. For example, if the subsequent employer terminated the employee after the employee requested a maternity leave of absence, that termination does not cut off the back pay damages. If an employee quits her subsequent job, but that job was substantially equivalent to the job she held with the defendant; her back pay is not cut off.
So when a subsequent job is lost whether by way of termination or voluntary resignation, it is mistake to automatically assume that the the back pay is cut off. Under certain circumstances, the back pay will continue on.