Failures happen. So do mistakes. We design planes to be resistant as a result.
There are many engine issues that would not be visually apparent. Metal fatigue in the turbine blades, a bolt that’s the wrong size for its hole. Aviation has seen it all.
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.
This description is rather nonchalant but how can this possibly happen!? May this be related to the age of the aircraft?
"Ground observers reported hearing the sound of an explosion like bang, smoke and saw the debris falling down."
I mean, an engine about to explode sounds like the kind of thing that a routine pre-flight inspection should be able to catch...