Pressing the Windows key will do that as windows takes the focus. If you press Windows Key + D all windows get minimised and you'll be on desktop

Are you on laptop. ? ( you have seen laptop usre use balance powerscematic mode and other powerscematice mode with max output to games aka max pefomance, and same goes with have powercord in and there is juice it need to run it.

ps.
Some windows notification can do that.

and Focus mode most be enable today so Main app have resources, you can read very old post with this where we turn it off, reason is when they made it back then our pc run better without it, things has change today so many task in OS and apps that take resources away from Main app.

You can find many user that notice alot of task running worst time doing game, your laptop/pc need to be a moster pc if backup is running and you do not notice such. ( most think they have world best pc, thats not the case for many steam users. )