Something to try:
Open System Settings, then Keyboard.
Click on Keyboard Shortcuts, then click on Services (from the left-side list)
Click the > at "Pictures"...
Do you have a keyboard shortcut listed in "Set Desktop Picture"?
(I don't think that will randomly "change" the wallpaper, but it's a step at a time to figure out what might be happening, and might have a key combo that you remember