You don't know that. The mechanic could have been following a P&W procedure that did not account for some wear in the engine. Is that mechanic even a United employee or P&W or a contractor? The mechanic could have installed the wrong part. It could have been a totally random part failure. I can go on. There's too many possibilities to speculate at this point. Reports say the engine was vibrating before the cowling came off. That could be caused by many factors. It could have been a bird strike. The plane in front could have lost an engine or tire and this one sucked up FOD. We just do not know yet. The plane landed, the engine will be inspected.
I don’t know that, but with the present lack of information I was venturing a low information guess simply because the aircraft is otherwise very reliable in all its variants. In any case it looks like groundings are currently being ordered only for 777s with the P&W engines because they uniquely have hollow fan blades.
