Having upper traps that “take over” on an exercise is a common experience, and one I’ve encountered many times in my training career.
9 times out 10, when I assess their technique on exercises, they do their exercises like this: immediate trap contraction and shoulder elevation.
I’m exaggerating for demonstration sake, but this “trap dominance” is not a mysterious muscle mystery, it’s a biomechanical problem.
Many people do their back exercise by pulling with their hands, not initiating with their elbows.
This causes the upper traps to immediately kick in, and they take over the exercise. This can happen with any kind of pull, as well as lateral raises.
What the solution?
You need to lighten the weights and relearn the technique for these exercises.