As a pure guess, I would say:
1) This bunker is not original to the course
2) The depressed area to the left of the bunker is a drainage area that remains quite wet for long periods of time, and so:
3) Someone had the ingenious idea of highlighting the edge of the fairway with a bunker, possibly thinking doing so would enhance the difficuty and beauty of the hole, and when faced with having to deal with the drainage area, wouldn't let the idea go and forced the bunker on the hole using the retaining wall, with predictacble results.
Of course, that is all conjecture.
As far as the rules go, generally such a retaining wall would be considered integral to the course, thus no relief without penalty. The committee could declare it an immoveable obstruction ( I would vote for such a designation for daily play) and a player would get free relief for interference with stance, swing, and lie of ball (but not line of flight.)