The AD doesn't have to prove or substantiate or even describe how  they feel she's not fulfilling her job description.    That's the point.  When you're hired for a job, even that one, you don't own the job in perpetuity.   You serve at the pleasure of those who hired you, within the bounds of whatever contract was made at your hiring.

If the AD is breaking that contract, that's another matter.   But it seems that she is arguing that her employer has no right to ever replace her.