I would bet they were designed as bunkers and the golf committee and/or construction team didn't follow through. Or depending on what year it was built and how close to the depression that was, they were probably filled in relatively quickly.
Get a 3' soil probe and go down into the middle of them. They tended just to be top soiled over and you can find a substantial layer of sand down there. I was told by the super that the bunkers at my old Ross course could have been dug out, refilled and be right back into business.
Running around with a soil probe on an old course is a lot of fun.
You will learn a lot about the green pads too.
This is of course based on the course not having been worked on too much.