The stargate network must undergo periodic correlative updates in order to compensate for stellar drift. Carter had theorized that once every couple of hundred years the gates dial each other and automatically transmit the new coordinates that apply to each gate address.
Felger and Carter believed that the Avenger virus might have triggered an automatic internal protocol in the DHD, causing it to initiate an automatic correlative update which transmitted new scrambled coordinates to neighboring gates until the entire stargate system had shut down. However it was later determined that it was not Avenger, but rather Ba'al's modification of the virus, that was responsible for shutting down the gate network.
In fact, Ba'al had acquired his knowledge from Nerus, a Goa'uld in service to Ba'al. Nerus had been studying the stargate system for many months, searching for a way to help defeat the Replicators, and he had stumbled upon a way to disable the correlative update program so that a stargate could be moved without transmitting its new position to the rest of the network. Ba'al later put Nerus's research into practice when he attempted to rebuild his power base, after the defeat of the System Lords, by stealing stargates and moving them to new worlds where he could reestablish his empire. Once the gates had been repositioned and the program set, it would have been impossible to detect, however SG-1 recovered the stolen stargates before Ba'al could complete his plan.
Cross Reference: Avenger, Ba'al, DHD, Jay Felger, Nerus, Stargate
Episode Reference: Avenger 2.0, Off the Grid