We have a few users in our environment having the same issue. I haven't got to troubleshoot it a lot, but one thing I plan on trying is upgrading to the mac that have an old version VMware, to VMware Fusion 7. I read somewhere that had a focus stealing bug in it. Could you post a list of Applications the Mac has and the ones that are currently open. Also, what model and OS are installed?

This can happen from some background applications running. Since they are faceless apps, they don't really bring focus to anything in particular. While background apps should actually never steal window focus (I believe Apple's guidelines spell that out for developers) sometimes they are poorly designed and will do this.
You can try disabling any and all login items, then log out and log back in, so only applications you generally launch are running and see what happens. If it stops it may be one of your login items doing it.
Keep in mind that many menu bar items are actually applications and not true menu bar items like the ones Apple ships with the OS. Sometimes these are the culprit for this behavior.

I was experiencing this on my work machine. It turned out to be an older version of the Google Drive app.

I found this Python code that helped: https://discussions.apple.com/thread/5470393?start=29

I'm having the same issue in my environment. mac OS Sierra High 10.13.6

We don't run Symantec but we do currently have McAfee. I'm currently testing to see if Chrome is maybe the culprit too.