It's your CPU.
The GTX 1080 is a very powerful card, and your CPU just can't keep up. Your CPU is trying to send all those draw calls to your GPU and it just can't send anymore at a certain point.
Let me put it this way.
Say your CPU can't send more than 13,000 draw calls to your GPU. Now on low graphics settings you are sending those 13,000 draw calls and you are getting 130 FPS and your GPU utilization is at 50%.
On the other hand, lets say you are on max settings. Now your CPU has to only send 6,000 draw calls because the CPU is now waiting for the GPU to finish work on those draw calls therefore you are getting 100% GPU utilization
Now in those 13,000 draw calls, a single draw call has very little data and geometry/detail, therefore it gets processed faster by the GPU, and your GPU is like "Yo bro CPU, come on, give me more work. I'm just standing here waiting for you". Therefore you are getting more CPU utilization than GPU.
On the other hand with the 6,000 draw calls, a single draw call has much more data in it and much more geometry/detail, therefore it takes a while for your GPU to finish work on that draw call and you CPU is like "Yo GPU, come on man, why are you taking so long. I'm trying send more draw calls to you, finish faster" therefore you are getting more GPU utilization than CPU.
Hope that helps