Mac goes back to login screen randomly - Mac pro 2013 -

I'm not sure this will fix but it will surely help to isolate if your drivers are causing an issue.

Please reboot into safe mode and see if you can reproduce the same issue.

Incase you need info about safe mode: https://support.apple.com/en-us/HT201262

Note: Safe mode prevents your Mac from loading certain software as it starts up, including login items, system extensions not required by macOS, and fonts not installed by macOS. It also does a basic check of your startup disk, similar to using First Aid in Disk Utility. And it deletes some system caches, including font caches and the kernel cache, which are automatically created again as needed.

1)

See if you have a panic report (file's ending in .panic).  If you have more than 1, please post a couple as the differences can be very useful:

Look for the Kernel Panic reports at:

Finder -> Go -> Go to Folder -> /Library/Logs/DiagnosticReports 

<http://support.apple.com/en-us/HT200553>

The panic report should have ".panic" in the file name.

You can put the panic report in an "Additional Text" box when you Reply

If, AND ONLY IF, you have difficulty posting via "Additional Text"

then try posting to PasteBin.com, and give us a PasteBin URL link.

<http://pastebin.com/>

2)

Please post the EtreCheck output as a "Reply" to this thread (you can use the FREE mode)

<https://itunes.apple.com/app/etrecheck/id1423715984?ls=1&mt=12>

Click on the Share icon and select "Copy Report"

And then Paste the report as a "Reply" to this thread using an "Additional Text" box.

EtreCheck is a tool that helps Apple Support Community volunteers debug problems without any access to the troubled computers. Debugging problems can be a difficult task even when the machine is in front of you. Attempting it via a discussion forum is extremely difficult. EtreCheck is a great help in that regards.

.

If, AND ONLY IF, you have difficulty posting via "Additional Text"

then try posting to PasteBin.com, and give us a PasteBin URL link.

<http://pastebin.com/>

3)

You should also try an SMC reset and an NVRAM reset 

Reset the System Management Controller (SMC) on your Mac - Apple Support

https://support.apple.com/en-us/HT201295

How to reset NVRAM on your Mac - Apple Support

https://support.apple.com/en-us/HT204063

An SMC reset and an NVRAM reset are not harmful, and sometimes they actually are useful.

Unfortunately, I am moderately good with .panic files, but no so much with application crash reports.

No .panic files at least indicates your Mac did not panic and restart.

Unfortunately, I do not have any clues about why your Mac goes back to your login screen.