Had in issue once where something similar was happening.

Turned out that the Sharepoint admin had set some sort of full catalogue or scan job to run just before the backups were due to run.

This was not a SQL job, it was something in the SP Admin centre. I believe it was somehting similar to a reindex job and it was writing (very heavily) to a table.

Sorry I'm not being very technical, Sharepoint is not my forte 🙂

I put on my Authority hat and forbade them to set it running untill after the backups were done!