Windows 2008 Server R2 turns itself off

Solution 1:

Usually this is due to automatic updates. Both the 109 and 13 Event IDs can indicate a reboot. Look for update messages in the event log from WindowsUpdateClient, for example:

...

Also, if someone pressed the power button that can initiate a graceful shutdown with many systems.

Solution 2:

Probably not the answer but a contribution. Found the same error in a couple of virtual servers, after an uptime alarm, and could see all services being stopped nicely and found that 109 was an entry after a planned reboot from Citrix service event ID 1074

Log Name:      System
Source:        USER32
Event ID:      1074
Task Category: None
Level:         Information
Keywords:      Classic
User:          SYSTEM
Description:
The process C:\Program Files (x86)\Citrix\System32\Citrix\Ima\IMAAdvanceSrv.exe 
               has initiated the restart of computer <COMPUTERNAME> 
               on behalf of user NT AUTHORITY\SYSTEM for the following 
               reason: Application: Maintenance (Planned)
 Reason Code: 0x80040001
 Shutdown Type: restart

Solution 3:

Have you considered the possibility of someone simply hitting the power button ? That will cause exactly the behavior you described.