you are right that employment is, as you say, at-will, the timing between an employee's union activities and their termination is going to be quite relevant so long as they were a well performing employee. Whether that is the basis for a law suit however will depend on the law in the employee's area. For a union, negotiation power arises out of how "disposable" the unionized employees are and how many employees are a member of a union. Once unionization is achieved, it gets harder for an employee to terminate en masse at will without bringing the union into it.