Full Screen games minimize randomly

I have a windows 7 64bit system that has started to minimize full screen games. There are no obvious pop-ups, windows or alerts that caused the problem.

How can I see what is taking priority on my system in those instances?

I would pull up the Task Manager, sort by CPU usage and just watch it while using the computer to see if something weird pops to the top of the list.
Keep a few different windows open and see if they also lose focus occasionally.

Bitdefender did this on my system for Steam games. It made itself a big PITA until I whitelisted Steam.

Thanks for the responses! I'll give both a try. This has been a very frustrating problem.