I'd always assumed..probably incorrectly...that part of airline logistics has influence on how seats are open, sold, and available on a flight for this very reason. I thought they tended to both open up selection from say, the back first and halt the front when the forward seats were getting full, and that they encouraged the GA's to start filling special requests and standbys from the back forward. Guess I was wrong.
I can see how this would happen on a carrier with open seating like WN, or on a smaller jet where it matters more and two empty seats in the rear can significantly upset the weight and balance..but on an a321, on a mainline like AC, it is surprising. I Suppose it is a combination of unique factors..light load to start, lots of no shows who were assigned to aft seats, light cargo..