Genetics...
The only thing I do is deads, but my traps were pretty large(in comparison to the rest of my body) even before I started deadlifting....
I only wish my pecs and bi's grew like my traps
Don't forget that your traps go down to the middle of your back, and all the way across to your shoulderblades. You can stimulate the muscle best by training all parts of it...not just that little hump up by your neck.