Why is she so aggressive?

I work with this girl who is SO aggressive. She has to be right all the time and will be hostile towards everyone to the point where no one wants to question her or stand up to her. She treats people like they're children and likes to control every single thing other people do.

I think she's a little scared of me because I have fought back before. But I can't figure out why someone would be like this?
Why is she so aggressive?
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