In Alberta I recall another annoying habit. Flat and straight four lane, cruise set at 120, someone overtakes in the left, just to settle in front of me and proceed to slow down. Pass them, resume cruise control, and it happens again.
This happens a lot. My take is that most people don't use cruise control and therefore don't follow stable speed patterns on long trips. Sometimes we end up passing the same vehicle five or six times. And then there's the tractor beam effect described above when a zombie driver just sticks to your speed. Floor it for a bit, then reset to your cruising speed and the zombie fades out in the distance.
Maybe the driver is getting blown.