Adam,
Those bunkers on the right of 13 were apparently added in the 80's as containment bunkers, as players were spending too much time looking for balls after losing their tee shots right.
As for 12, the original tee was well left and short of the current tee. There was a large tree left, and the hole actually played as a dogleg left (great old picture in Mal Elliott's book, actually), and the goalposts weren't as they are now, but much smaller. I would imagine the left bunkers were put there for the same reason as the right bunkers on 13, containment for balls heading toward the tall grass.