One fan or compressor blade becoming unbalanced for any reason, or foreign body damage to a blade causing a harmonic vibration and domino effect on other blades, or over RPM. So many things, their mechanics will know with just a cursory look and data recorder. The pilots did a helluva job setting it down
Excerpt from below link
There are only two solid rules regarding jet engines; The airflow through the engine can never exceed the speed of sound and the maximum RPM isn't far from the point where the engine starts slinging compressor blades.
https://answers.yahoo.com/question/index?qid=20070209211033AA1JPw3
> “The pilots did a helluva job setting it down”
The crew and the passengers are so lucky to be alive.