AT&T says the mass outage that affected about 60,000 of its customers Thursday was caused by a software update. Specifically, “the application and execution of an incorrect process used as we were expanding our network” was to blame, the telecom company said in a statement to ABC News. Outages were reported with other cell carriers as well (though Verizon and T-Mobile explained that their networks did not actually experience outages and that any outages seen on Down Detector were likely a reflection of their customers struggling to connect with customers on other networks who were experiencing…