It looks like of of the fan blades detached and ripped cowling off. Such engine defects are usually not detectable during routine pre-flight inspection, for that you need to disassemble whole engine, check bearings, do x-ray inspection of turbine to look for metal fatigue or micro cracks etc.

Two years ago there was very similar incident, also Boeing 777 with PW4000 engine: http://aerossurance.com/safety-management/ndi-failures-b777-...