Ok my crap reason is here why Tannu wants to stop HumaYan engagement šAs HumaYan marriage will take place with Nikhat-Imran marriage so Tannu doesn't want this engagement so that Nikhat-Imran marriage gets delayed and She can hope to get Imran back. As she is pregnant with Imran's kid according to her which Imran denying..
Also another reason can be to take revenge on Raziya, as Raziya insulted her, and she wants Huma engagement doesn't happen so that people can talk bad about Humeira that she's sick,ill and what not. And Tannu don want to leave this chance from her hands
So by this she manipulates Zoya and makes that mess on engagement
i'm with her if she does so on first reason. As she will get her love also won't interfere on Asad's life... If she gets her right by evil way, its okay as Imran refused to take her back..