"Dual Action" was a trademark of National Detroit Tool, one of the first in the US to market an air powered random orbital sander. Their original tool was known as the "DAQ" and was the standard against which knock-offs were measured. Whether the name implied the fact that spinning motion combined with orbital motion comprised the "dual action," or whether the eccentric with a tang that turned the tool into a grinder was the second action in "dual action" might just be lost to history. Most of this is pre-Internet. Personally, at this date, I find DA, dual action, and random orbital to be distinctions without any meaningful difference, other than the fact that many of the old 2-handed tools had a "grind mode" described above which was limited in its usefulness, as far as I remember.