This is merely more of the existing tagging along the HS2 route which seeks to differentiate between the land which will become railway (landuse=construction,construction=railway) and that which is currently construction temporarily and is presumed destined to be returned to be useful farmland (assuming the ground level is returned to 'natural').  I don't know where it started.  It's imperfect and could be construction=farmland but that is not the purpose of the construction.  No construction= tag/value at all could probably be better but then validators would bleat.