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 threat…
There is still a civil way to accept a resignation, with proper timelines and transfer of responsibilities. She offered to resign in some time due to some demands, and the immediate termination seems to be related to an email unrelated to those demands. This sounds a lot like a sleazy move by Google to couch an immediate firing as a resignation.
So looks like Timnit sent an e-mail to the group which wasn't OK in the eyes of the higher management, then sent a strong mail about her demands to the same management and used "I'll resign otherwise" card to force her way.
The management, agitated from the first e-mail, didn't buy it and sent her home. The way they did is neither elegant nor polite but, she pushed a lot to get this response it seems.
This is my understanding of the issue and I may be completely wrong but, this is what I was able to decipher.