Looking at pictures 2 & 4 from the BBC you can clearly see the cowling in picture 2 is fully intact (qantas logo visible) however after the application of foam it has become fragmented, (picture 4) so it's possible this was due in part to the firefighting operation on the ground and may be the pressure and thermal stress on contact with the foam jet being applied.
Foam into an engine thats still hot can be exciting no wonder there was cowling bits being picked up afterwards on the ground and collected into a pile.