Your Mac keeps changing your desktop wallpaper because you’ve selected the Dynamic Desktop option in the settings. To fix the issue, go to System Preferences > Desktop & Screen Saver > Desktop and untick the Change picture box.
Another likely reason the desktop background keeps changing on Mac after a restart is an external display bug. There’s no way to address the issue other than to disconnect the display before shutting your Mac down.
The constantly changing desktop background could also be a software incompatibility already addressed by Apple engineers. Therefore, if the two previous fixes don’t help, update macOS.