Adam,

Funny that you should say "they resemble chocolate drops"....often when Don would be working he would lose track of time and get very hungry. He had a problem with low blood sugar and would begin to hallucinate about food. More often than not those hallucinations would be "chocolate drops"!

Evidently, they managed to work their way into his work.

AFC,

Debris mounds would seem to make sense.

Perhaps you could take deep core samples and solve the mystery.

Adam,
    If they are perfectly shaped, then perhaps they were the original spectator mounds. I don't know about the shape part, and behind the green certainly doesn't sound like an architectural feature, so I'm still inclined to think debris pile. If they are out of play it sounds like it would be better to just get rid of them as it sounds like they must look odd.

I think Ross put most of his debris piles mounds on the side of fairways. It is always riskly to put something that will settle that close to greens.

I think it was merely a design choice.  Ross was certainly aware that many scottish courses set in dunes, and it was probably not unheard of him to try to replicate those as a design feature, although any earthmoving with horse and scoop certainly would have trouble replicating the scale!

As mentioned, he may have wanted to protect a tee, divert water, or just to give a green a different look.  

It may have been a technical problem.  They tried to balance cut and fill on site in many cases. If they "eyeballed" a green grade (since they had no true grading plans) and set it a bit low, they had to get rid of the dirt somewhere close, and a greenside mound is a logical addition.  If it was a fill pad, maybe they left the site for a while, and the horse kept working, and brought in too much fill causing the same problem.  

There is a elephant buried in the back right corner of the 17th green at Holston Hills.  Perhaps Messrs. Doak or Stiles can shed some light.  Makes no sense to me.

Mike

Below is a picture of the 18th green of course 2 at Olympia Fields, taken in 1921, with a pretty big chocolate drop at the back left.  The hole is terrific, reminiscent of the old 18 at Oak Hill with the green set back of a hilltop.  It is a Willie Watson original, and the hole is now number 9 on the South Course, added in 1946 when most of the rest of courses 2 and 3 were sold off.  The green contours are still there (the green has shrunk a little at the back, and will be restored), but the choco drop is no longer extant.  There was another cool one on the 15th hole of what is now the South Course.