Maybe she wasn't ready. Why does it have to be sass to let the umpire know you're not ready. Not saying it wasn't sass, but it's possible it wasn't. Is there a time limit to be in the box? If so, that's when the ump should have taken action. If not, he could still have reprimanded her.
She wasn't even close to being out of the box. It was either spite or incompetence—neither acceptable in that situation.
If the ump had to motion her to get in the box then she was probably taking too long. She should have stepped in, not looked at him and said "I'm not ready yet". That, to me, is disrespectful. I agree that she was not out of the box at all, so then the question becomes why did he call her out after that double - seems obvious to me that it was out of spite - and no I don't agree that it was the right call, but had she not sassed him, I doubt she would have been called out - in my silly opinion.
The fact that she then said to him "you suck" maybe shows a little more of her personality. Who knows, maybe other things/issues transpired between her and the ump during the course of that game that we didn't get to see also.