Why do some co-employees are too bossy?

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I have this co - employee who was first employed than I am,15 years or more senior than I am. At first, she was too nice, we take lunch at the same time, she tells her stories, etc. After a few weeks or a month, she has changed. Subsequently, another secretary was employed and became close to me. The senior employee stopped taking our lunch with us, she tells stories against us, she tends to order or command us things that she supposed to do. Everybody hates her attitude.