The way the art here, Cass is SO not done. She'll be around.
From a "good" writing standpoint, unless we end up learning what's going through Cass's head, I'm gonna say that if this a case of "bad" writing and not part of a lingering plot thread, I'm going to say perhaps her reasoning is just, she's really sore, like Tim, about loosing Bruce. He had just adopted her, yay acceptance and WOOSH BAM RIP OMEGA BEAM - OOPS ignored by everyone. I would imagine she might be a little bitter that oh this new father figure for her, is BOOM gone. People grieve in certain ways.