Short answer: it's "legitimate" disk usage by a MS tool, just like the MsMpEng (MS Malware Protection Engine) it's scanning your disk regularly.
How do I get to that statement?
Well, I have had the same behaviour on my Win 7 machines and did some digging. First, I used the Resource Monitor and found CompatTelRunner.exe reading what looked like all of my disks/files, like e.g. a malware scanner does. During that time it was also writing to a directory ( C:\windows\temp\compattelemetrylogs ). When it was done with the disk work, it started showing network activity on the upstream (matching what is mentioned in an Avast Forum Entry). I checked the above directory and found confirmed what the file and directory names hint at: it seems to be "Compatibility Telemetry Information". That means that MS seems to be scanning all PCs that have the update installed (i.e. all that are Win10-compatible), collecting information about all installed programs and features (read: about "everything"), and then sends at least part of that information or a status report to MS servers (I have not network-screened it... yet).
The frequent re-releases of the update can mean many different things, like compatibility updates, scan rule updates, anything, really. However, I don't like that MS scrapes all of my machines (and everyone else's, too) for information and does not tell me about it. Even if that information may be helpful or even necessary in providing the seamless upgrade path to Win10.
If you don't want to go to Win10 on that machine, you should consider removing the upgrade.