It's not that OneDrive is modifying files "as it syncs"; it's that it involves Sharepoint, which adds certain metadata to documents that it handles, and has probably always done so; here's a stackoverflow question about it from 2010:

http://stackoverflow.com/questions/2762841/how-can-i-stop-sh...

EDIT: I see that 'ppog has already mentioned this

https://news.ycombinator.com/item?id=7624293