Our trapezius or “traps” divide into three sections. They have have radically different shapes, fiber directions and primary movement functions. Yet, this does still behave as one muscle. In particular, when our lower traps are weak or inhibited, our upper and middle traps will try to assist.
One of the primary functions of the upper traps is to elevate the shoulders. However, one of the primary depressors of the shoulder are your lower traps. The only other shoulder depressor is serratus anterior. So, any attempt to “help” from our upper traps tends to elevate our shoulders and lock them in a battle they cannot win.