My guess is she felt like her head didn't want to come back to her. So, if finding out your purpose for the past 20 years or so was a waste of time, you'd probably get pissed off, too. Pair that with the person responsible for it basically being close by, and it's no surprise she went on the offensive, too.
I haven't read the manga, though, so I'm not really sure where things are going next. Is it done or still on-going?
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