I am not an expert in the foundation's bylaws, and the only thing they said publicly is "The Rust Foundation Board of Directors also wishes the best to Ashley Williams, who will be leaving the post of interim executive director "[1].
I expect that it was a group decision and not a sole one. But again, my point is not about how that happened, my point is that it has happened, and what that means for the state of the organization as a whole, that is, that leadership is concentrated in a single organization.
1: https://foundation.rust-lang.org/posts/2021-06-25-announcing...
