As always, #avherald has the neutral description of what really happened with that Boeing (do ignore the comments there, though). TL;DR a panel that can be reconfigured as emergency exit in high density configurations dropped out. https://avherald.com/h?article=51354f78&opt=0
This is the picture that explains the situation a lot better. Unfortunately most news reports did NOT show this. What you see here is that a panel cleanly separated, indicating a mechanical problem. This panel covers an optional emergency exit in the frame. So most probably that panel somehow got loose and separated cleanly. Now to find out WHY that happened on a brand new plane. I guess we will know soon.
According to other sources, the plane did report pressurisation problems on earlier flights. This adds to the suspicion that the outside panel/emergency door became more and more loose over time and finally separated itself. My educated guess is that this problem will be traced to a mistake during manufacturing.
Ouch. "FAA grounds more than 170 Boeing 737 Max 9s after Alaska Airlines panel blows out" — seems it is a systemic problem now. Quality control issues at Boeing? https://www.cnbc.com/2024/01/06/boeing-737-max-9-grounding-after-alaska-airlines-door-blows-midflight.html