I use CPU rendering (AMD Ryzen 9 3900X 12 cores). Sometimes when rendering an animation or 32K image using tiles, blender stops using all CPU cores on random frames or tiles. And it will start using only 10% of the CPU (2 cores). This happens until the next frame or tile. On the next frame, blender starts using all processor cores again. I didn’t find any patterns, this happens on random frames or tiles every time. There are no errors in log.
There are situations when this happens during frame rendering. Blender renders a certain number of samples on all cores, and at some point there are only 2 cores left on a random sample, and so on until the end of the frame rendering
Welcome 
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…maybe also the operation system decides which cores are assigned to which application… so that the user is able to do some interacting with the system…
So maybe blender starts already the next frame while others are still rendering on the previous and so only some are available when starting the this frame… but then then the (over)next one starts with more while this frame only uses less…
IDK… i do just have two cores… 
Also: The auto detectionon your system might be… not so good ?? See also Docs-Blender Manual: render settings … and there is a differnece in mutli-core usage for rendereing and simulation.
This is probably because the entire process of rendering does not use 100% of the CPU.
If some cores stay 100% and others don’t work, check the settings
So a couple of assumptions. In theory this is using a latest version of Blender and you are doing a Cycles render. Not sure why you aren’t using a GPU, but that’s a whole different discussion.
Between the rendering of each frame, Blender needs to setup the scene, etc and it’s possible that some parts of that (depending on what is in the animation) will only use 1-2 cores. Also, if you are denoising, then again, there’s a chance that will also only use 1-2 cores.
However, for the actual rendering part (with Cycles) Blender will use as many cores as you can throw at it and push them all to 100%. I’ve seen 64 core Threadrippers (with hyperthreading) so 128 threads do a Blender render and all maxed at 100%.
Of course there could be other factors at play, without a sample scene of something that drops to 10%, it’s all partly guess work.