What could be a reason for a Proxy SG to un-join a domain

Hi;

A Proxy SG un-join a domain suddenly and the users get this error message, when trying to authenticate to any site:

"An unrecoverable error was encountered: "The IWA direct realm encountered an unmapped error code, contact your system administrator."

Exception: Internal error 

What could the un-join reason be. A failed health check for example? I had the impression that this does not constitue a reason for loosing domain trust.

Kindly

Wasfi