[ethanp61]

She made demands and gave a resignation date if those demands were not met. Google said that they could not meet those demands and accepted her resignation but because of her conduct (emailing hundreds of Googlers), the resignation date was made immediate.

[danpalmer]

I believe on currently available information she did not give a date, but said she wanted to "discuss her exit timeline".

Also it's likely that her contract allows Google to provide her pay in lieu of her working out the end of her contract, so they are probably able in general to move dates like this (not commenting on the other aspects of this situation though).