> what changed in Boeing's design, testing and/or manufacturing processes
My theory is that this is not limited to Boeing or even aircraft design, it's a much deeper and systemic problem affecting all kinds of fields. We've had a lot of industrial accidents lately.
When aircraft manufacturing was an emerging industry there were tons of undocumented safety margins and "slack" in the design and production pipeline.
Over time, the beancounters start optimizing stuff, so these undocumented safety margins are eroded in the name of efficiency/profit (and sometimes even documented safety margins too).
Furthermore, workers back in the day had a much better life when it comes to purchasing power (especially when it comes to property), and so could actually "give more fucks" about the job than they do now which is a compounding factor. You used to get a lot of implicit quality assurance back then which you don't get now.
We've now reached a stage where these undocumented safety margins have been eroded enough that it actually starts to cause issues, and the safeguards that are supposed to catch them aren't good enough, either due to 1) they've never been good enough but just weren't really needed before or 2) they too have been eroded in the same way for the same reason.
This also applies to software - software quality nowadays has gone down the drain for the same reasons, and even brands that were built on quality and polish (Apple) are now churning out shit (see the endless calls for another "Snow Leopard" bugfix-only macOS release).