After Installing El Capitan I Will be Logged Out Frequently MacPro late 2009 with 3 monitors. OSX 10.11.1
Will run great on 1 monitor without screen/system freeze or logouts. At times it is an instant logout, other times, the primary monitor freezes, or is filled with weird graphics. NVIDIA GeForce GT 120 512 MB
Began with 12 G Ram and upgraded to 24 after having problems with El Capitan. Suspected OS memory leak. 
Then did full erase and OS install, but before restoring my data, created new user and used Safari to test. On each monitor, a Safari tab streaming news videos. Ran for hours, but when I started using the system, it had video problems and locked up or logged out. Don't remember which.  
Then restored all my backup data and settings.
Activity Monitor shows full 24 G installed, but the Memory Used and Cached Files never go above 12 G
TechTools 8, all test pass. Memory, video, temperature, fans etc. 
Apple Care 2nd level tried to no avail.
System performed flawlessly until El Capitan install
System now is totally unreliable.
I installed 10.11.2 yesterday and am hopeful that fixed it for me as it has many others. I'll update. 
It appears that 10.11.2 has fixed it. This was the first time I had an update go bad, thankfully. Communication would go a long way, but results are always appreciated. I still feel they (Apple) are the best horse in the race. BTW here is the Apple Support thread on this; https://discussions.apple.com/thread/7252069?start=0&amp;tstart=0
From 
https://support.apple.com/kb/PH21782:


  OS X El Capitan: Set your Mac to log out when not in use
  
  You can set your Mac to automatically log out the current user after a
  set period of inactivity. This helps ensure that your information is
  secure when your Mac is unattended.
  
  
  Choose Apple menu > System Preferences, click Security &amp; Privacy, then click General.
  Click the lock icon to unlock it, then enter an administrator name and password.
  Click Advanced.
  Select “Log out after... minutes of inactivity.”
  Set the amount of time before the user is automatically logged out.