I would imagine overload did cause the lack of signals... at least to start with. Police have emergency access to phone personal that could then shut down the needed towers quickly (2-5 minutes tops) by simply pulling the tower(s) from the network. After the crisis or whatever passes the tower(s) are added back into the network and will start working immediately. Probably was a combination of these two things.
I'm telling y'all: There WERE NOT huge numbers of Texas Tech fans using cell phones from at least 7:30 p.m. to 8 p.m., and yet, all I could get was "svc unavailable". I'd like to hear from a FReeper who was at a Big 12 football game tonight and was able to use their cell phone during this time.