rsync to internet server connection breaks unless VPN is used, is Comcast throttling me?

For several years now I have been doing nightly backups of internet hosts that I lease onto a workstation here at home, via Comcast internet.  About 30 days ago these backups started failing because the rsync command being used would see its connection to the remote hosts get dropped, i.e. a "broken pipe" error.  I did all kinds of troubleshooting, looking at the versions of rsync being used on each end, command line options, debugging logs, etc.  I could find no explanation.  Finally, I tried installing a VPN client on the local workstation, doing the backups running them over an encrypted VPN connection.  Now everything works fine, backups over the last three days have been just as successful as they historically had been.

Given that one of the benefits of using a VPN is that it prevents an ISP from inspecting data packets traversing the network connection and applying throttling rules to that traffic, I'm wondering if that's what was breaking my rsync connections when I tried to run then over an unencypted connection.  Should I be filing a complaint with the FCC against Comcast for throttling my traffic?  Or should I be looking for some other cause for these broken pipe errors?

Has anyone else seen similar problems with Comcast internet?  I know they claim not to do throttling, but do they do it in some other guise, i.e. data classification or priority rules?