It seems likely The Night King is using that specific stretch of land as a rallying point before The White Walkers cross The Wall into the North of Westeros.
With supposed months passed since the first sighting to now, it might be that The Night King has finally amassed an army large enough to make a real run at this world domination thing and is finally ready to unveil his strategy to getting around the wall.
As seen in this week’s trailer for the next episode (featured on the next page), there are a TON of White Walkers on the march and getting all those bodies resurrected and congregated en masse to that one area had to take some time.