Possible moisture is interacting with the wallpaper glue underneath. Is this an exterior wall? Is the joining another room, maybe a bathroom or unheated space?
Have you tried to wash the stained areas with a clean white cloth and some water? If some of the stain is lifted with a simple washing then it isn't moisture coming through. Could be something like air fresheners or other chemicals reacting with the wallpaper directly.
Just a couple ideas, hope it helps.