why would my OneDrive suddenly be processing over 40,000 changes?

Same here now, reprocessing over 300,000 files across computers with no changes to the files.

Same here.

More than 270.000 are being "processed".

No idea what Onedrive is doing.

The problem is not that you have no idea what OneDrive is doing, Igor, but that Microsoft has NO IDEA what they're doing. In my case, I am paying for their (dis)service. Must also say, that Dropbox has no problem, whatsoever, and I am using their free account. A switch to Dropbox is eminent!

I had the same issue a couple of days ago. Suddenly millions of changes being processed and going down super slowly like 2 every second and changes I make during that time weren't syncing, API calls to edit files weren't working.

I decided to reset OneDrive. With that, it seemed to process changes much faster, like hundreds per second instead and it finished within two days I think. After that, I had a few remaining issues:

1. A folder was renamed from "x" to "x-USER" (x being the original folder name and USER being my PC name), as if there is a conflict but only "x-USER" was present while "x" was totally gone. When I tried renaming it back to "x" on my PC, OneDrive would rename it back to "x-USER". When I renamed it on the website, it gave me this error "request was cancelled by event received. If attempting to delete a non-empty folder, it's possible that it's on hold". So what I ended up doing was moving it out of my OneDrive to another folder on my PC, renamed it back to "x", deleted the folder on OneDrive website and copied it back to OneDrive.
2. Same as 1 but not my local folder, a shared folder from someone else that I added. I tried removing the folder from my drive, which was successful, then re-adding it from the shared list, but OneDrive said the folder is already added. So I had to ask the sharer to remove me, deleted the folder from my local PC, re-share the folder to me then I was able to add it back to my OneDrive. However, when it synced to my local PC, instead of being a normal folder that you can see files locally, it became an internet shortcut where when you open the folder, the browser opens to that folder on OneDrive.
3. Two video files were stuck in a download loop (even though those files are already on my PC). It would download and complete and re-start downloading again. I resolved this by moving the file out of OneDrive, deleted it on OneDrive browser and restored it from the recycle bin on OneDrive browser. This time it downloaded once and finished.

Now no changes being processed, files syncing and API calls working as before. Been considering moving to another cloud service for a while now because I've been getting OneDrive sync issues all the time and Microsoft support doesn't seem to be able to help (getting conflicts with word docs and excel spreadsheets or files won't update) and then this issue really pushed it over for me.

I have exactly the same problem as you mentioned under 1.