Any ideas on why this could be happening? I'll get calls from people saying they can't login and when I check, everything in their AD account is fine except the "User logon name:" under the "Account" tab is blanked out.
You probably want to turn on auditing for changes to Active Directory, then when it happens again you can consult the audit trail to see who or what made the change. The group policy you're interested in is:
You can set a GPO on the Domain Controllers OU to turn this on. See these articles for details: