Technical people who are intrinsically motivated (typically the best and most productive people) often are the sort of people who find working with other high caliber people to be an important and highly valuable part of their work experience. When good people leave because the company isn't treating them well the result is a cascade effect, because every good person leaving makes the working environment worse. So more good people leave. Then for everyone left behind there's extra work and a struggle to reacquire lost institutional knowledge, and everybody finds that executing on projects is more difficult and results in less success.

Because of the nature of software development (no development cycle is ever really identical to the previous one) it can be difficult to conclusively or objectively say that the capabilities of the organization have been reduced, or that the working environment is poorer, because these things require subjective judgments. But many people have enough experience to make those judgments for themselves, to feel the organization getting worse and less capable, and this can create a kind of pressure for people to find other options elsewhere even if not everyone understands why on a conscious level.

Sometimes, but it can be a bit more complicated.

Perhaps it is accurate to say the most mobile, unattached people tend to go first.

Perhaps the more loyal people stick around longer -- this puts a positive spin on it.

But you could also say that the most opportunistic people go first -- putting a negative spin on it.

Also, I think it depends heavily on what you mean by "bad". As a company culture deteriorates, sometimes it is the people who have been there the longest that don't notice the decline (see https://en.wikipedia.org/wiki/Boiling_frog)