The American heart association changed ABC (Airway, Breathing, Compressions) to CAB (Compressions, Airway, Breathing) because when the ABC sequence to perform CPR was performed, people would spend so much time trying to open the airway that there was a delay in chest compressions making rescue breathes harmful. With the new CAB sequence people initiate chest compressions sooner and ventilation is only slightly delayed.
Hope this helps!