In the book what she throws are pillows. Buttercup (Prim's cat) reminds her of Prim. Her intent is to chase him away. She feels first anger then as the emotion overwhelms her she breaks down. But they end up comforting each other. The passage from the book: "It was the waste of a trip. She's not here," I tell him. Buttercup hisses again. "She's not here. You can hiss all you like. You won't find Prim." At her name, he perks up. Raises his flattened ears. Begins to meow hopefully. "Get out!" He dodges the pillow I throw at him. "Go away! There's nothing left for you here!" I start to shake, furious with him. "She's not coming back! She's never ever coming back here again!" I grab another pillow and get to my feet to improve my aim. Out of nowhere, the tears begin to pour down my cheeks. "She's dead." I clutch my middle to dull the pain. Sink down on my heals, rocking the pillow, crying. "She's dead, you stupid cat. She's dead." A new sound, part crying, part singing, comes out of my body, giving voice to my despair. Buttercup begins to wail as well. No matter I do, he won't go. He circles me, just out of reach, as wave after wave of sobs racks my body, until eventually I fall unconscious. But he must understand. He must know that the unthinkable has happened and to survive will require previously unthinkable acts. Because hours later, when I come to in my bed, he's there in the moonlight. Crouched beside me, yellow eyes alert, guarding me from the night. It's a shame they couldn't reproduce this in the film. The cat was filmed separately, so was never in any danger.
I guess, for dramatic purposes, they had to have Kat throw something noisy -- and also exaggerate Kat's frenzy, because Jennifer has been playing "emotionally frozen" up to this point.
It's been established that Katniss and Buttercup didn't get along, and his presence was the last straw for her. It's the living memory of Prim's death.
She threw things at a cat because it's a cat. If Prim's pet was a dog, that scene would never have been written that way, in the book or the film. In America, dogs are worshipped as gods. Cats are fair game.
The most ridiculous thing about that scene was that the cat didn't get the fvck out of there. That must have been some genetically engineered super cat to stay so cool!
...Buttercup had survived and Prim hadn't. I forget how they explained it in the book, but how did Buttercup get there? The last we see of the cat is during the air raid in the previous film.
Maybe because "cat" is first syllable in "Katniss"? (getting ready to dodge tomatoes, stage hook, rocks, and empty soda cans as I prepare to run offstage for that corny answer)