When the ControlLogix was being designed in the late 90's, the general internal name for the platform was the Automation Controller. You can stil find remnants of the AC name deep within the documentation. Bonus points if you find references to RSLogix 5000's development codename.
Best guess, then, is that .ACD was Automation Controller Data or Diagram or Developer. The .L5K and .L5X extensions came after the product was officially called RSLogix 5000.