OSX Mountain Lion: Automatically restarts after showing screen saver a certain amount of time
Ok well. After getting more and more annoyed, I finally figured it out.
Apparently I enabled the automatic logout after 60mins of inactivity in the security settings. I really don't know why/when I enabled this and wouldn't consider myself a noob ;)
For anyone else looking for a howto on this:
http://support.apple.com/kb/PH4014 (I found this page and thought: that must be it!).. It was.