You answered your own question.
Not only your rotator cuff muscles, because that motion is not in the anatomical range of motion for the supraspinatus, infraspinatus, teres minor or subscapularis. The motions they perform are scaption abduction, internal rotation, external rotation, and decceleration movements. Not a 360 degree (or variation of a 360 degree) movement.
Also, the humerus sits in the shoulder's joint capsule (gleno-humeral joint) and has cartilage and the labram inside. These act as lubes and guides for various movements of the shoulder. Circumduction IS one of them, BUT not intended for over moderate weight baring movements. You're pretty much grinding out that cartilage, which can lead to arthritis, bursitis (from muscle guarding to accomodate the loss of joint capsule function) and just over all pain. Later in life, you will get decresed range of motion, leading to a 'frozen shoulder'.
To make a long story short, I wrote a paper on this a couple years ago and I had a full page of citations that say no to circular shrugs. Your traps are involved in this movement, but there's too many aiding muscles to attack the traps directly.
Keep shrugging, heavy dumbell shrugs, hold at the peak and slow down. Your form is what gets those mountain traps. Dont do them quickly, do a full range of motion (ie. all the way to the top, slow back down, drop your shoulders, take it all the way back up, etc.). Try doing them sitting down. Dont crunch your neck down to a shortened position when shrugging, A LOT Of people do that and count that as a shrug, it happens a lot in the later reps. So take a weight you can normally do for 15+ reps and try them slow, controlled and concentrate to the essentric movement and getting that full inferior to superior range of motion. For barbell shrusg (which never have worked well for me), switch up your hand grips. If you do them in the anatomical position (palms out), try a set or two with your palms in, shock them is what Im trying to say.
Best of luck, traps can be a bitch to get to grow, but once you figure out what works for you, they generally grow at a quick rate. YJ