I think perhaps it is that on some primal level we tag the information as important. There is a compulsion to absorb it, to take advantage of a rare opportunity to see - what? A vision of your own end, perhaps? A comprehension of the depth of misery to which the world can abruptly sink? Escape routes to store away for later use?
Perhaps it is simply that it is a peek through the manufactured and peaceful world to a kind of reality that seems firmer and more fundamental. This world is physical. I am made of thinking meat. It could be turned to hamburger in an instant, or some sudden neurological glitch could turn the thinking part psychotic, or aphasic, or violent, or, again, simply dead. Everything we construct ourselves to be in our identities and in our society is transcended in the train wreck. How could we not be transfixed?