Recurring pop up window iCloud

The icloud window is popping up randomly on the PC running Windows 10. Just started a couple of weeks ago, nothing new for software here.

Why is this happening? Why?

I really don't wish to uninstall and reinstall anything, so please do not tell me I need to do that, I know it most likely works, but I do not wish to use very valuable time to do this. Not to mention the resync of 100,000's of thousands of photo files to the server will take forever.

I just want to know why it is happening, what is causing this to happen, so I can diagnose the fix afterwards. Apple user since 1986, forced to use PC for business apps and AutoCAD; yes, I know, but the workarounds drove me nuts for many years at first...