Kate Mulgrew was extremely pissed for a long time and it really is quite understandable, she was excited about the show and that all changed when Lien was dropped and Ryan was brought on, but she had 4 more years to go.
Not to the point of making a coworker's life miserable, simply because she took a job. Jeri did not create the role. Jeri did not force herself into getting hired.
Kate's behavior isn't reasonable adult behavior in any workplace.
And yes, it happens. Still doesn't make it right.