Animation using Cycle CPU use

Hi All. I’ve come back to blender now that I can use my GPU again (5700xt) and have been trying to render out an animation using cycles and HIP.

I’m getting some strange behaviour and I’m just wondering if I’m not understanding the way rendering works, but also cannot find an explanation using my Google-fu.

I’ve included an image below of my task manager, as you can see my GPU is doing the rendering work. It them seems to pass something off to the CPU as CPU and memory use seem to take a hit. Note while this is happening my VRAM is sitting unused.

The animation I am doing is using a lot of particles (comet trail) so I was wondering if it may be something to do with that?

I can render the classroom scene and my GPU is used exclusively. And because the animation is just rendering individual images for a sequence (only 570MB mem use at most) should it not be doing the same?