I wondered when the buttock-baring sense dated from and I see that OED has a 1964 cite (moon, v. 4a)
1964 Princeton Alumni Weekly 7 July 34/3 All we had was..abortive efforts at mooning the Yale team in their dugout.
It’s interesting that it differentiates this from buttock-baring intended to insult or shock (moon, v. 4b) for which it gives a 1965 cite
1965 N. J. Eaton Stud.Current Adolescent Slang (M.A. thesis, Univ. Massachusetts–Amherst) iii. 36 Gloss. Moon,..to expose the buttocks. syn. Throw a moon.
Perhaps it supposes that those attempting to moon the Yale team intended it as a gesture of respect!
There are several earlier senses of the verb moon of course, eg the equivalent of sunning oneself, to wander about listlessly, to daydream. I was unfamiliar with the chiefly Australian sense of hunting by moonlight though.
This was prompted by something I read in an article on the unpleasant Ms Roseanne Barr in the LA Times.
The storm escalated when Barr married Arnold in 1990. Their outrageous off-screen behavior was gold for tabloid covers — they once mooned a camera at a sporting event with their rear-ends —
With their rear-ends? Is there any other way? Although with Arnold and Barr perhaps their front-ends would have been equally effective.