Just for info:
B747 engines were designed to break off in the event of a crash landing, the engines are suppose to flip over the wing (possibly doing a one and a half turn twisting pike). After the El Al incident it was decided that perhaps the engines were better staying on the wing and all B747s went through a mod programme. For this incident I'll speculate on the pylon cracking until it couldn't hold the engine any longer. I would not be suprised if the 'pins' were still on the A/C.
Good news though that there were no reported casualties.