SQL Server 2008 Authentication mode changed on its own
The chances that the Authentication Mode changed on it's own is pretty near zero. More likely, it got changed by accident.

This is a change that requires a restart of the SQL Server service in order for it to take effect. What likely happened is that the Authentication Mode was changed sometime since the last restart (which could have been months ago)--then, when the instance restarted, the change "suddenly" took effect seemingly on it's own. 

This particular configuration is stored in the registry, so if you have OS backups of your server, you could do some research to go back in time to try to determine when it changed.