Why does my Windows 10 wallpaper keep changing?

If your picture is constantly changing, you probably have the “Slideshow” option selected. Assuming you are using Windows 10, your desktop background, also called a wallpaper, is determined by your personalized settings.

Why did Microsoft change my wallpaper?

This is usually caused by account syncing, though that can and does go wrong on a regular basis, it is just theme syncing, no personal data will have been transferred . . . Power to the Developer!