When something like passing trains or heavy traffic keep cracking plaster, or wallpaper corners in a house, you will not succeed in stopping the movement, but you can hide it.
Cut the wallpaper neatly in the stressed corner.
Nail molding into the corner, but attached only to one side. The other side will be left to float. If you are doing this on a newly painted wall, paint right into the corner first, then install the molding. That way, when the molding shifts a bit, you don't reveal an unpainted portion of the wall.