Zira was pouncing to attack and kill Simba and Kiara intervened to save him. Then the two ended up rolling down the cliff. Zira was not doing a single thing to Kiara. She meant to do it to Simba.
As the official kovu addict and for the sake of sanity.... I must agree with the above statements, Zira sought revenge on kiara for the ruining of her plan, the "betrayal" of Kovu, and the defeat of her idea