最新网上分析，这个应急门是Alaska airline改造过的，有可能是他们自己没有安装好

What's not mentioned in the article: The 737 MAX 9 has a rear-mid cabin door. Alaska removed the door and installed a plug (panel) in its place. Airlines do this when a rear mid-cabin door isn't required for their seating configuration.

 

That plug is what blew out on Friday.

 

Other people in this thread are claiming this is a materials problem, citing carbon fiber as the culprit. These people should be ignored; they're telling you a bunch of nonsense that they've invented in their own mind, no doubt after convincing themselves that 10 minutes of internet research has given them expertise on aircraft design and materials science.

 

We won't definitively know the cause until the NTSB investigation completes, but a very reasonable guess at this point is that the plug was improperly installed.