This morning, I tried to upload two large-ish video files (185MB and 159MB) from my M8 to my OneDrive over my secure home wifi. The files appeared to upload, then my phone locked up and rebooted.The files did appear on my OneDrive a few minutes later.
This evening, I checked my data usage and saw it was WAY higher than it should have been. The culprits were two sessions from this morning, one for 185MB and the other for 159MB, both after I started uploading those videos.
So, it appears that my M8 not only didn't upload the files over wifi as I had told it to, and it crashed when it uploaded them over cellular, which I have explicitly disabled in my Storage Sense and OneDrive app settings. I don't see anyone else with this problem when I search, either here or on the interwebs in general. What exactly happened?