Hi Jeff,
Unfortunately, I haven't heard about any similar cases and didn't manage to "google" one of them. I have only one assumption - since TFS uses cached password, it might be something goes wrong on this side. When the account gets locked again, try to check the cached password for this user's account in Control Panel -> Stored Usernames and Passwords.
Also, I would recommend that you copy this query to the TFS support team. They as vendors should have more understanding of what is going wrong and should be able to assist you:
https://visualstudio.microsoft.com/team-services/tfs-support/?rr=https%3A%2F%2Fwww.google.ru%2F
It would be great if you can share the solution once you find it!