Sophos UTM restarted itself, for no reason

i would check the system.log and boot.log 

support.sophos.com/.../KB-000036784

Did it update automatically? 

is there a out of the box way to do this at all ?

You can either wait for Sophos to push it to the UTM automatically, or you can download the Up2Date file and upload it to manually update the UTM.

Either one normally requires a restart and does this automatically after a version update has been applied to the UTM.

I have mine set to download the updates automatically, but not apply.  I'll usually wait a few weeks to months before applying anything to make sure the sky isn't falling.

Also, before any updates are applied, the UTM system is imaged (it's a vm under proxmox). Takes all of about 15 minutes to do so, but guarantees full revert if there's any issues. I suppose I could snapshot too, but I try to align my updates with quarterly imaging anyway.