how does auto reboot happens?

For "no reason?" my machine is rebooting.

last reboot brings lines like this:

reboot   system boot  3.13.0-24-generi Tue Dec  9 18:57 - 19:09  (00:11)    
reboot   system boot  3.13.0-24-generi Tue Dec  9 12:59 - 19:09  (06:09)    
reboot   system boot  3.13.0-24-generi Tue Dec  9 11:07 - 11:35  (00:28)    
reboot   system boot  3.13.0-24-generi Mon Dec  8 13:52 - 01:20  (11:27)

With the pseudo-user reboot.

I need to know who is calling/activating this pseudo-user reboot? why? and how to fix/prevent that?

I am still researching, but my machine may suddenly reboot, so I have to post now!

linked: Why does a grep reboot the machine?, about IO problems