The simplest explanation is that the Bun team decided to try rewriting Bun in Rust because they had some frustrations with Zig. I haven't seen any evidence that there's more to it than that. It's easy to get sucked into all the internet drama, but that only exists because Zig's author lost his cool and wrote a salty blog post, not because of any conniving on Athropic's part.
I agree that that's the surface-level, superficial, zero-thinking description of what happened.
You're right that I haven't spent much time thinking about why a project that I'm not involved with, and that is of only moderate general importance, has decided to switch from one programming language to another. Anyone who's convinced themselves that this is all part of some elaborate scheme is almost certainly overthinking it.