I haven't heard about the Northern hemisphere's movements! Have you got a reference?
GPS may well have shown this movement (or it could have come from radar or lidar surface mapping through other satellites, which is more likely). Part of GPS operation includes monitoring errors, and a systematic error where something is regularly and unexpectedly changing will stand out a mile. Or a millimetre, which is just as good.
Nothing is fixed in GPS; all it does is coat the planet in a mesh of signals that can be decoded to provide an arbitrary code that everyone understands relates to positions on the ground and to the compass. But the ground is moving and the poles are moving, all in three dimensions. Where there's a discrepancy between the mesh and the real world, it'll be spotted and, in time, coped with.
R