I had a similar issue in 2014-2015, where suddenly my SSD filled up until it ran out of space and memory. I recorded it and called Apple. Their technical manager (who seemed quite competent and very customer friendly actually had no clue and started investigating via remote access etc.
The one thing I had done before that may explain it, was that a previous apple support told me to delete the iCloud cache folder to refresh things. While that solved a prior issue, my system registered my iCloud content as not updated and started downloading massive GBs (that was already supposed to be synced. The Apple Tech Manager and I discussed this as a “possible explanation” and that it is unwise to do any manual stuff with iCloud preferences and cache folders. In theory, it should not act this way and Apple techs were not aware of this consequence but we agreed that it was the most likely explanation. So if you have removed or moved any folders (system configs or just your own content) that has not been registered for iCloud sync, this may cause such a problem. I have not had this again but I also changed my iCloud content to such that will less likely need organising etc.
–> iCloud has a cache folder that is being synced and one explanation could be that when this is deleted or moved, or renamed etc. the syncing gets cranky and fail to recognise that you already have the content, so it downloads everything again.
–> In theory, the cache folder ought to be safe to delete and Apple Techs were very surprised by this issue even at senior level from our remote session
–> My solution was to turn off and resync everything again, and deleting pref files. iCloud is a cranky baby