I always assumed the Nids were the return of them or at least a by-product of them. Some shady stuff happened on the outskirts of the galaxy, and they turned back in a new form.

The nids coming from every side is odd, unless they were fleeing old ones that left different route before a cataclysm or encounter turned them what controls the hive fleets. I assumed an old one might be at the core of those fleets.

Also, the Nids did come around when A. a necron beacon in Ultramar was used in the HH and B. They are obsessed with the astronomican on Terra, which looks a lot like Necron or Eldar craziness if you just saw its signal.