MacOS Catalina 10.15.2 - Constant switching to Finder

Recently, using my iMac Retina 5K, 27 inches, Late 2015, no matter what app I am using, it suddenly switches to the Finder. It means I can no longer type in any app. Just now, it happened in Safari. When I was typing the title of this post, I couldn't get past the word "Constant," before it switched. I know this because I immediately went to the menu bar at the top (it is always hidden), and it showed I was in the Finder. I had to click on Safari to bring it to the front again, but it still happens over and over.

I tried force-quitting the Finder, and that didn't help. I haven't downloaded any new apps in two weeks.

What else can I try? It is infuriating!

Thank you,

Greg