MacBook Air logs off automatically since Yosemite My MacBook Air mid-2012 model running OS X Yosemite logs off automatically without any prior warning when I am working on it.
This has been happening since the latest update, to Yosemite. 

The logs from the moment of logout are here: 


http://pastebin.com/embed_js.php?i=u3TaFSWH


To be more specific my Air has been connected to an external monitor both times and has been charging both times too!

Similar problems have been discussed on Apple Support Communities:


https://discussions.apple.com/thread/2742595?searchText=automatic%20log%20off
According to this Apple support article: Set your mac to log out when not in use - installing Yosemite turns on the "log off after 60 minutes of inactivity" feature. 

To turn it back off, click on the "Advanced" button at the bottom of the "Security and Privacy" window and uncheck the option. You should be good to go!