We simply don't know that,yet. It could be a maintenance issue with whoever Southwest contracted. From someone fucking up closing the engine cover properly to the use of counterfeit parts (actually a larger issue than most people know - it has brought down airliners in the past).

It also could be an issue with actual Boeing certified spare parts or maintenance protocols- Something breakers earlier than expected,some spare parts are not as good as expected, the procedure is overly complicated,etc.

And of course it still could be an issue with actual Boeing works - something is built badly and now, after thousands of hours finally kicks the bucket - there have been cases where repairs brought down airliners decades after them being down (badly),the same is absolutely possible for engineering or assembly mishaps. If that is the case Boeing would be really fucked.

Anyways: There is a reason NTSB reports take months to years until they are finished. It is ridiculous how many people "know" what the reason for this was.