This wreck doesn't appear to be the result of "human error", but occurred when one train (carrying mostly covered hoppers with either grain or soy) derailed -- and then "fouled" the track where the tank train was passing, derailing it, too. It happens now and then -- not too long ago there was a train derailment/collision in Bridgeport, Connecticut, where an eastbound Metro-North commuter train derailed after a rail joint broke beneath it, and ended up fouling the track on which a westbound was approaching.
With freight, could be a number of causes -- perhaps an axle journal overheated and broke. It's been very cold there, which increases the chances of a rail or joint bar breaking underneath a passing train (like with the MN accident above).
It's railroadin' -- no matter what you do, things like this can happen. Not much more to do other than clean things up and get back to business...