While it is hardly possible to do little more than guess I'll put out a couple of thoughts and maybe you have or have not thought of them.
You might have a genetic predisposition to have a large neck. I do too; my neck is bigger than my biceps or calves. I never really did any special exercises for it either. I only remember that when in middle school, there was a neck exercise machine where you could sit down and move a pad with your neck. I was able to do 200lbs on it, more than anyone, while being much weaker than at least half of the kids in other exercises.
Also since you made lots of gains within the past year, if your neck is genetically predisposed to grow, it would, with very little stimulation. The shrugs might have been enough.
Maybe you also, unconsciously, have a habit of tensing your neck when you strain to do any exercise. That way your neck would get much more stimulation than you would think.
I personally think that it is great to have a big neck. If you (lets hope not) get involved in any accident, your neck muscles will give you extra protection that a person with a regular neck would not have. It would minimize the trauma from whiplash. Second, powerful neck muscles allow you to take a punch much better without your head getting rattled (compared to if your neck was weak). Thirdly, I think a strong neck gives people the impression that you are in shape. It is the only part of your body that is still visible when wearing a business suit or a coat.