As much as I would like to dogpile on Boeing right now, that particular airplane has been in operation since 2015. This would likely be Southwest maintenance issues rather than Boeing.
It's systemic by this point, and I feel that it's not only the fault of those "pesky MBAs running Boeing" anymore.
On top of not necessarily being the manufacturer’s fault, I don’t know that these events happen more often than usual.
There are known unknown structural issues with 737 NG's (-6xx to -9xx non-MAX) beginning around 1995 that have never been resolved because they were assembled poorly and built with substandard "approved" components by subcontractor DuCommun but aren't so serious that they would necessarily cause an immediate, catastrophic failure. Instead, there have been an increase in fuselage damage and failures under heavy load circumstances such as hard landings, runway overruns, and potentially during extreme turbulence. Al Jazerra made a feature-length documentary about it and the tales of the whisleblowers, but because of the predominant anti-muslim bias in America, it has been largely ignored and dismissed by the mainstream. Even Wikipedia includes biased conclusions that Obama's DOJ rammed down the throats of media and countermanded the NTSB's investigations and statements. [0]
0. https://en.wikipedia.org/wiki/Ducommun#Faulty_parts_dispute_...
So no, the most plausible conclusion is not a structural failure but that maintenance opened the cowling and then failed to properly latch it and that was not caught by the pilot who did the walk-around.