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! Done for the day. We will be moving to another solution! It just filled the hard drive on a laptop that was connected to the account. An enormous number of files that weren't necessary on the device and not flagged to be local. It's all messed up now! I've been paying for extra space for 13 years. I would think that when it comes to updating services that it could be done in a professional way and not so hush hush.

12 pc, 140.000 changes.
We can't work!
It's awful!

I am having the same issue with OneDrive as others have reported in this thread.

Just experienced this issue on over 7 different computers on different operating systems and in different physical locations. Two more computers at one of the locations were not affected. The only thing I could find that might shed some light on this is that the computers that had OneDrive blow up on them were all on version number 25.031.0217.0003, which (interestingly enough) is not listed on MS OneDrive release notes page as far as I can see. None of the computers/accounts in question are signed up for the "insider program/early release/beta" versions (and you couldn't pay me to sign up for that if recent events are what "tried and tested" service looks like). All 7 computers had to be completely unlinked from OneDrive and SharePoint and reset, which took a few days and was EXTREMELY disruptive.

EDIT: Forgot to mention -- all computers in question only had a few folders from our library syncing (our full library is VERY large) and when OneDrive exploded, ALL the library folders were suddenly selected on the computers in question and OneDrive was trying to sync the ENTIRE library to each computer. I understand Microsoft is trying to push the "files on demand" feature, but that doesn't work for us and I'm sure there are plenty of other companies out there that it doesn't work for as well. So, Microsoft, this is where an option to interrupt the syncing process and switch off folders currently trying to download would be very nice. The only option we have is "pause," which is useless and locks us out of the settings to deselect folders we don't want syncing, or "stop sync," which also removes the ability to deselect unwanted folders. Who thought that was a good idea? If we try to unselect any folders while OneDrive is trying to sync oh, say an ENTIRE library, we get an error message that states it can't do that right now. Super helpful... Sorry, not sorry. Need to vent somewhere as this experience was probably the WORST OneDrive experience I've ever had and we've been using OneDrive since 2018...