How could this grievance have been avoided?

Housekeeping employees in your hotel have filed a grievance against you. They say that your hotel failed to abide by the conditions outlined in your collective bargaining agreement with their union. According to the employees, the housekeeping supervisor has under-scheduled personnel in her department so that she and her assistant can make up the rooms themselves—thereby saving the department money. Employees filed a “performance of bargaining unit work”

grievance asserting that the manager was taking away their work and attempting to eliminate their jobs.