What causes "cam walk"?

The timing chain has nothing to do with cam walk. The rearward cam thrust is created by the load of the oil pump! The direction of the helical gears on the cam/pump drive want to drive the cam backwards when the pump starts pumping oil....which should be always. The lifter offset is designed to counteract this force, but the minimal contact area of the lobe/lifter is not a lot so it just 'helps'. Roller lifters do not have this restraining force, so the cam could move forward slightly under the right conditions.