It is one of the causes, because Spirit have had numerous serious quality control lapses they've tried to hide (including as trivial and as serious as failing to drill holes properly). But Boeing itself has the same types of issues, so while it will give them more control, there won't be a fundamental change unless Boeing manage to fix their culture, which is highly unlikely in the short term. Especially while they're preparing for union contract negotiations with multiple crucial unions that have them by the balls.
The people who made the dumb mistake that led to the Alaska Airlines blowout that could have been deadly quite easily were all union members. They worked around the system, removed bolts, did some work, forgot to put back bolts in. (I say they because Boeing, negligently, has no records of who did what, so the whole team of 20 people is responsible).
The main theme for Boeing in the past few decades has been lowering costs. Be it by forcing the hands of unions to lower salaries and benefits, or moving production to non-union places (787s in South Carolina), or offloading as much design and manufacturing work to subcontractors as possible, spinning off "unrelated" business such as manufacturing. Most of those were absolute disasters.
It feels like Boeing divested itself of Spirit to try and avoid the responsibilities of having to manufacture various components, but the awful culture was retained at Spirit and probably made worse by the priorities of its private equity owners.
> Spirit have had numerous serious quality control lapses
Boeing spun out Spirit to be able to fire all the experienced union employees and move the work to a state where they could hire non-union people, all part of cost-cutting.
