i am suspecting that speeding caused the derailment. it is pretty obvious the curve has a very small radius, and normally has a very low speed limit. if the train was running below the limit, the derailment would not have happened, and even if it did happen, the damage would not have been so disastrous.
update: holy smoke! the speed limit is 80km/h, and while Renfe insists that the train was traveling at 180km/h, but other Spanish government sources are saying the train was traveling at 220km/h at the time of the accident. Either way, it's above twice the speed limit. this is serious, even worse than the Chinese HSR accident, hope it will not reach the level of German HSR accident.