"Lloyd E. Sponenburgh" writes:

From the rather small niche of black powder historians, the story
comes this way:

Anyone who physically handled cannon balls or powder was known as a
"ball monkey" or "powder monkey". The term "powder monkey" is still
used today. "Ball monkey" seems to have been lost.

And where is this etymology traced from?
If you can find me a single contemporary citation, I'll believe you,
but until then, I still say "back formation"