I saw this happen with the London bombings. A colleague was almost in tears because her son lived in London and she couldn't contact him on the phone. It turned out he was fine, as approximately everybody else in the city was too.
The world is full of real people, with real understandable concerns for their families and friends.
Not only that, but it isn't about the number who were killed anyway. It is about the probability of your friend being in that location at the time of the disaster which is much, much higher.
No one thinks 'I wonder if Jack was killed, he was at that park today'. They think 'I wonder if Jack was in the CBD, he works nearby!'
Statistically validated concerns or not, the very real effect of this part of human nature is that the phone infrastructure goes down and that sucks for everyone.