While we have been told that for 9 hours she did not answer many questions, she did state that she helped Trump with little white lies. So after that 10 minute conferring with attorney and her response, there is a lot of time to kill. I suspect Hicks had been informed of much of what the Mueller team has collected and what they already know. An education. Regardless of her January 31 prior decision, a new paradigm was presented and new decisions had to be made. just my opinion.

she met with Congressional Intelligence Committee not Mueller.

Which really puts a spin on her time in the WH after that.

I am guessing she either wore a wire or continued to give Mueller info. and her sudden notice of resignation might be connected to a heads up from Mueller.
As good a theory as any others going 'round.

then after she told the truth about lying for trump, he called her stupid. Apparently, that was the final straw.

Probably it finally has sunk in that she could be facing a few years in prison for statements like "those emails will never get out".