Googling this I see it's a common issue although I've never seen it on my rig.
One user reported this fixed it:
I turned off Windows Error Reporting, and problem went away.
Start > Control Panel > type reporting > Choose how to report problems > Never check for solutions > OK
Okay, after the reboot, it still triggered. I will try the McAfee Getsusp Tool to see if there is anything awry with the installation. If that is a negative, I will eliminate Windows Error Reporting, per instructed.
Lost focus at 10/21/2014 10:06:02 PM due to C:\PROGRA~1\COMMON~1\McAfee\Platform\McUICnt.exe
Lost focus at 10/22/2014 1:06:01 AM due to C:\PROGRA~1\COMMON~1\McAfee\Platform\McUICnt.exe
Lost focus at 10/22/2014 4:06:01 AM due to C:\PROGRA~1\COMMON~1\McAfee\Platform\McUICnt.exe
Just wanted to quickly mention that this is still occurring. I recently went through an out-of-state move and did not have time to troubleshoot. However, I will be reevaluating ASAP.
Okay, I at least reproduced it again:
Lost focus at 1/6/2015 11:12:45 PM due to C:\PROGRA~1\COMMON~1\McAfee\Platform\McUICnt.exe
Lost focus at 1/7/2015 2:12:45 AM due to C:\PROGRA~1\COMMON~1\McAfee\Platform\McUICnt.exe
Lost focus at 1/7/2015 5:12:45 AM due to C:\PROGRA~1\COMMON~1\McAfee\Platform\McUICnt.exe
Lost focus at 1/7/2015 8:12:45 AM due to C:\PROGRA~1\COMMON~1\McAfee\Platform\McUICnt.exe
The program I am using to monitor this is called Focus Taker.
Now that I have proven it is still the same issue, I will try to reread through thread and attempt the suggestions where I left off.
I have decided that I need to rule out some sort of an issue with my machine. Therefore, I will try to reproduce this on another machine that has Windows 8.1 and McAfee Antivirus as well. If I can do that, I will then uninstall McAfee Antivirus to determine if the issue goes away. On the other hand, if I am unable to reproduce it on another machine, then I know that something is awry with my McAfee installation on this computer... which would seem very weird to me but nonetheless possible, I suppose. This may take me a few days or weeks.
I have successfully reproduced this issue on an old laptop. It is running Windows 8.1, 32 bit. The same issue occurs precisely as my primary system. The timing is the same as well, every 3 hours.
The next step is to uninstall McAfee from that laptop to see if the issue disappears. What I am wondering is if McUICnt is inadvertently hiding the identity of the true culprit. Removing it will determine the case. If I remove McAfee, the issue disappears, and my McAfee installation validated correctly (based on former diagnostics), it must be something in the McUICnt programming that is causing the problem.
We will probably know in a couple of days. I plan to go ahead and let the laptop sit another 6 hours to see if it continues to have issues.
Looking online for general hints on "application stealing focus" I came across this, is it any help?