I can only assume he saw a man with a gun and knew something was about to happen so decided to film it. Wouldn't be my first reaction to a gunman, I'd be running as fast as my legs could carry me.
It's all just so bleak at the minute, every time I turn on the news, it's another gunman on the loose, another city bombed to nothing, just constant never ending atrocities that we humans inflict on each other. We're supposed to be the most evolved species on the planet and yet we are seemingly incapable of living in peace.