The answer is given in the film itself (spoilers below, so stop reading now).
He was posted there as punishment for murdering Admiral Archer's dog.
Kirk did not smile. “You’d be surprised at what I know. What did you
do to get yourself posted to this vacation paradise?”
Scott grew animated. “I got into a debate with my instructor on the
issue of relativisitic physics as they relate to subspace travel. He
seemed to think the range of transporting a, say, roast turkey, was
limited to a few hundred kilometers. So I told him not only could I
beam a bird from one planet to an adjacent planet in the same system,
which is no big deal anyway, but that if I were so inclined I could
actually do it with a viable life-form. Long-range transwarp beaming
is supposed to be impossible.” He snorted. “Difficult maybe, but not
“Says you,” countered Kirk.
“Says I, aye.” The engineer glared back at him. “My mistake was in
attemptin’ a practical demonstration. Unfortunately, for a test
subject I chose Admiral Archer’s prize beagle.” He shook his head
sadly. “Shoulda scanned the little mutt’s ident implant first, I
Kirk’s expression changed to one of surprise. “I know of the
admiral—and his dog. What happened to it?”
Scott looked away. “I’ll tell ye when it reappears. I’m convinced it
will, one of these days.”
Star Trek: Official Novelisation
The dog is, we learn