The email she posted [1] makes it pretty clear that she sent an email with some demands/conditions (separate from the one "to Brain women and allies") and said that she would resign if they weren't met. Google said they wouldn't agree to them, accepted her threatened resignation, and told her it was effective immediately.
Without knowing more context, it sounds like she tried to do some hardball negotiating by threatening to leave, and got her bluff called.
Quoting the tweets for posterity, link at the bottom:
> I need to be very careful what I say so let me be clear. They can come after me. No one told me that I was fired. You know legal speak, given that we're seeing who we're dealing with. This is the exact email I received from Megan who reports to Jeff
> Who I can't imagine would do this without consulting and clearing with him of course. So this is what is written in the email:
> Thanks for making your conditions clear. We cannot agree to #1 and #2 as you are requesting. We respect your decision to leave Google as a result, and we are accepting your resignation.
> However, we believe the end of your employment should happen faster than your email reflects because certain aspects of the email you sent last night to non-management employees in the brain group reflect behavior that is inconsistent with the expectations of a Google manager.
> As a result, we are accepting your resignation immediately, effective today. We will send your final paycheck to your address in Workday. When you return from your vacation, PeopleOps will reach out to you to coordinate the return of Google devices and assets.
[1]: https://twitter.com/timnitGebru/status/1334364732480958467