Could have been any number of factors really. If it were a single train incident, i.e. it didn't crash into any others, then the most likely cause would be excessive speed causing the train to leave the rails. Alternatively, the driver could have ignored a signal (that is often the cause of multiple train collisions) and hit ... well, in railway jargon, hit a set of trailing points set against his train. At slow speeds, it is sometimes possible to pass through those without derailing (although it fucks up the track and is not exactly a good idea), but at any decent speed, it'll likely send the train off the tracks.
To try to put that into non-gunzel speak ... one track splits into two at a set of points. When you are coming from the one track and it is about to split into two, then the points are facing you ("facing points"). If you are on one of the two tracks about to become one, then as you cross the points, they will be trailing behind you ("trailing points"). If you happen to come from one of the tracks but the trailing points are set for a train travelling to or from the OTHER track, then a piece of steel is going to be obstructing your train's wheels and the result is not going to be pretty.