I have this same problem. Running Windows 8.1 (formerly 8.0, upgraded it to 8.1) and it's been happening for a long time now. I have spent hours and hours troubleshooting with the Event Viewer, the Resource Monitor, and the Task Manager performance
tab all open and I can't see what's going on. I tried clean boot mode, I opened MSconfig and disabled all non-Windows processes and the issue is persisting. There are forums all over the internet with complaints about this and I am absolutely shocked
that the MS devs haven't picked up on the problem.
It's on the Blizzard forums for Diablo III, it's in the StarCraft 2 forums, it's on the Steam forums. It's on the Roblox forums. It's everywhere. Any person with even a slight iota of common sense could do a google search for "Windows
8.1 alt tab by itself" and see all the search hits where multitudes of people are describing the same problem, nearly all of which mention it happening after running Windows Update.
Additionally, there is a known issue that MS confirmed with the upgrade from 8.0 to 8.1 where it breaks the system restore functionality and I am flat out refusing to reinstall my OS for a problem that Microsoft has neglected to fix for the better
part of 3 months. The hotfix for rebuilding the restore functionality with command prompt just errors out for me.
No, I'm not going to provide links to all of these forums, no I'm not going to do any more troubleshooting on my end because I've already done enough. I do not have the time to do Microsoft's job. I'm disgusted with the service on this issue.