I'm not gonna pretend how the internals of Hololive function. Everyone thinks they can run the companies better than Yagoo, Fishman, etc. That's not to defend them at all, but we see the tip of the iceberg no matter what talents say, how well we think we know the company, a few poor decisions, etc. on how these things function as businesses.
Trying to split hairs dissecting the PR speak in these graduations isn't going to reveal any answers. They're all under NDAs and Hololive isn't a small place where there are defiant crashouts with talents airing dirty laundry. The ramifications are too high.
The contracts were open to be re-signed and couldn't be renegotiated for these girls that chose to leave. I'm sure they all have their own lawyers to deal with it. It could be money, demands, etc. but it's just not going to be known by the audience.
Western vtubing is a different in a lot of ways when these girls entered and rose to success. It's been enough time in the role to lose the enthusiasm, got a very comfortable amount of money, and most of them are in their mid-late 20s now wanting to move onto something else. Once one person chooses to pull the cord, it can also make it easier for others to follow suit.