What happened is that people stopped putting him in lane.
You see, they buffed his Q such that it gets double stacks if you kill cannon minions, big jungle monsters, dragon/baron, and champs. So suddenly, Nasus had a way to get decent Q stacks while avoiding the problems of lane matchups entirely. Not that his jungling strictly focuses on stacking Q, but the bonus is much appreciated. On top of that, he now gets 100 more cast range on W and E when he's ulted, which gives him better teamfight presence overall.
Then with Nasus free to run around in the evil forest, he could farm up some and also use Whither for ganks, which is pretty freakin' brutal even after the nerf on its attackspeed reduction. He also has excellent dragon control since you can solo it as soon as you hit lv6. His ult tears through it pretty fast. The S3 itemization favored his jungle as well, since aegis and locket pretty much have you set for life.
Top lane Nasus still has his place (and it's a bigger place than most people give him credit for, really), but his jungle is really strong right now since it can leverage his strengths throughout the game and not just after the 40 minute mark.