Cooling fans spin at very high speed and have lots of energy. If they catch debris or just come apart, they can toss chunks into critical areas, as this one did. The cooler can also be punctured and oil loss compounds the problem. One press report said an outside crewman was treated because he felt a piece hit him.
Modern fans are designed to contain the chunks, this one clearly was not able to do so. Reports indicated the newer design (that was not yet installed on this aircraft) has containment built in.
Such "turbine burst protection" is part of the new regulations, including all coolers and engines, but older designs do not have to refit to meet the new requirements.