That was a big mess. I've railfanned that area and the trains are always right on top of each other...not quite as literally as in this case, but it is easy to see how this wreck could happen once the first incident occurred. Very unfortunate for the westbound crew since they were probably less than five miles from their crew change point. The eastbounds were just on the job, so fatigue was not likely. After reading several sources, there are conflicting reports about the severity of the injuries to the crew.
Hard to speculate at this point, but these kinds of cornfield meets generally have only a few causes: excessive speed, disregard of signals, engineer impairment. I'm betting on the middle one. The loco data recorders will tell the story.