Not sure why or how
There's plenty of documentation to be found on the why and how, especially following the 737Max issues: https://team-fsa.com/insights/what-happened-to-boeings-cultu...
> Following the 1997 merger with McDonnell Douglas, Boeing’s robust culture eroded. Subsequent safety issues with the Boeing 737 have put the company under international scrutiny and underscored the profound impact of a weakened corporate culture. As Forbes aptly put it, “Boeing’s current travails about safety issues with the 737 MAX 9 can arguably be traced to the company’s weak corporate culture.”
Or read https://www.library.hbs.edu/working-knowledge/why-boeings-pr... for Harvard's take on the same situation.
The best and understated part about it is that the culture change was pushed from Boeing side, and at least some people from McD side of the merger were pushing internal memos warning about actions pushed by Boeing-lifer CEO exemplified in then ongoing 7X7 program (future 787)