Something is on or in the wall substrate and is reacting with either the wallpaper glue or the wallpaper. It is also possible that the reaction is on the outside of the wallpaper. This could be caused by many things like airborne pollutants (air fresheners) or insect excretions.
The point is; there are many reasons for the chemical reaction to take place. All that you can do is maybe wash the affected areas and see what happens.