Mostly his attitude. He's brash and disrespectful. Regardless of how he feels about it there are standards and rules, manners and simple things like speaking properly he must abide by now that he;s Kingsguard. It was part of what he swore to do when she took the oath and yet they continue to go against that. She believes him someone she cannot trust, and hence has no intension of keeping him after her comes into the thrones. She fully intends to take over after their father btw.